It's too easy just to say "Oh, some people are jealous." That's a non-answer. Whenever I hear someone say something like "Oh, I could never do that, I'm a jealous person," it sounds as nonsensical as saying "Oh, I'm a hungry person" or "Oh, I'm a tired person." Jealousy is an emotional response; to say "Oh, I'm a jealous person" and to let it go at that is to treat it as if it is some fixed, immutable thing we are powerless over, like saying "Oh, I'm a Western European person" or "Oh, I'm a dark-haired person." In fact, scratch that--many people seem to feel they have more control over the color of their hair than over their emotional lives!
Now, there are certainly plenty of people in the world who do indeed feel unhappy and insecure if their partner spends time with someone else. There are many reasons that someone might feel this way, of course; insecurity, low self-esteem, a feeling of being expendable or interchangeable, a feeling that one's needs are not important to one's partner, feelings of being marginalized or trivialized.
Some of these, such as low self-esteem, are internal. Low self-esteem in particular is a real bitch, especially when it comes to relationships; I've seen many people cling to their low self-esteem like drowning men cling to a piece of driftwood, refusing to give it up. It's self-reinforcing, because it creates a sense that you're not valuable and thare are many people in the world who are better than you are, so you best not let your partner be with any of them, or best make sure you're in control of the situation. The thought of giving up the low self-esteem is terrifying, because if you give up your low self-esteem, then it might be okay for your partner to spend time with another person--and you don't want that to happen, because it makes you feel insecure! Hence, you don't want to give up the low self-esteem, because giving it up means that you may face situations which...trigger your low self-esteem.
Some of these are external. There really are people who shouldn't be comfortable if their partners express an interest in someone else; there really are people who treat their partners as expendable and interchangeable, and who aren't concerned with taking care of their partner's needs. Many "free agents" in the poly community behave in ways that don't exactly inspire confidence in their lovers; some behave as if they barely recognize the differences between them at all.
Okay, so there's nothing new in any of that. We all know this already, right? Behave in a way that doesn't acknowledge the needs of your lover, and your lover may not feel secure in your relationship. Behave with indifference to your lover, and your lover may not feel secure in your relationship. Behave as if your lover is the flavor of the day--"Ooh, you're so cool, I dig you, I'm so glad we met, I totally lov--oh, look, potato chips!"--and your lover may not feel secure in your relationship. This isn't really rocket science.
But what happens if you flip that coin over and look at the other side?
There are people in the world--I've met more than a few--who have a strong sense of self, a robust sense of security, who are in partnerships with people who are sensitive to their needs and treat them well, yet who still seem plagued by insecurity in their relationships. I'm not talking about people who simply aren't polyamorous; there are secure people in healthy relationships who are just monogamous, and that's the end of it. No, I mean people who seem to be secure in themselves and have partners who treat them well, yet seem insecure in their relationships all the same. So what's the difference?
Conjecture: Putting your partner's needs first, putting your partner's happiness before your own, doing everything you do in your relationship for the sake of your partner, can also cause your partner to be insecure.
Reasoning: Now, this doesn't seem to make intuitive sense at all; if your partner is respectful of your needs and consistently puts your needs ahead of his own, it wouldn't seem like this should breed insecurity. Just the opposite, in fact; a person in a relationship with someone who has a consistent track record of making his happiness the most important thing should feel secure, right? But bear with me here.
Let's say Alice and Bob are in a relationship, and Alice consistently puts Bob's happiness ahead of her own. Alice genuinely wants to make Bob happy; in fact, this is her first priority in all matters great and small. Alice has always done everything she can to make Bob's needs her first concern. Could you reasonably expect Bob to feel secure with Alice?
I think the answer is "no." Every human being does have needs; a romantic relationship where one person's needs are important and another's are not isn't sustainable, even if it's the choice of the person whose needs are being neglected.
But it gets worse. If Alice has never made her needs or her happiness a priority, and has never stood up for the things she wants, then it's entirely possible that Bob doesn't understand her needs, and because of that has no idea how to make her happy. Alice's self-sacrifice backfires, because by not standing up for her needs, she has denied Bob the opportunity to meet them. Can Bob make Alice happy in their relationship? Bob has absolutely no way to know; he has no handle on what Alice needs...and indeed Alice herself may not have a handle on her needs! When the day comes that the relationship becomes unsustainable, when Alice must start considering her own happiness...what then? Bob doesn't have the tools to make Alice happy; if some situation comes along which DOES make Alice happy--even if it's a situation Alice herself could not have foreseen or anticipated--the Bob may very well lose Alice, and the poor guy never had a chance.
The dangers of putting your own needs ahead of everyone else's are pretty obvious, really. Being a selfish prick isn't a good relationship strategy, and I think most reasonably people can easily see why.
But the reverse--putting your partner's needs ahead of your own--is a dangerous game as well. There comes a point where you must stand up for your own happiness, and defend the things you need and want; if you do not, your partner may be left with no idea what those things are, and no idea how to make you happy.
Were I in a relationship with someone I did not know how to make happy, I do not believe it would be possible for me to be secure in that relationship--even if my lover did everything in the world for me. Reciprocity in a relationship is more than just fair; it's the very thing that gives the people involved the tools they need to make one another happy. It's very important for me that my lover stand up for her happiness, and be able to assert herself and ask for what she wants. If I know what she wants, I may or may not be able to provide it--but if I don't know what she wants, I don't have the most basic tools I need to make her happy, and if I cannot make her happy, I can never really trust that she will stay.