I've spent the evening watching season 7 of House. Here's what happens when I'm allowed to watch a fellow misanthrope in action...
Jealously and insecurity. I've heard a lot of people angst that their partner is 'irrationally jealous'. There's two possiblilities, and here's what you need to do about it. Firstly, take a really hard look at yourself. I don't mean this in a 'you're a complete dickbag, face it' kind of way, I mean that you need to take an honest look at whether you can eliminate yourself from this problem. Are you doing ANYTHING that might be exacerbating this? Anything at all? The hard part here is that you might have to look at the situation out of another persons eyes. Are you doing anything that THEY might have a reasonable case for thinking you're an untrustworthy sack of crap over? If the answer is yes, then grow the fuck up and fix yourself before you start picking holes. OK the thing you are doing might actually not be as bad as they are perceiving it to be, but if you can't talk to them about that then you have a much bigger problem than just insecurity.
OK so now that you've assured yourself that you are not, or at least are no longer, the problem and there is still a big old green monster sitting in the corner making everyone uncomfortable you can probably assume that your partner/friend/sibling/whatever has trust issues, probably because someone else (or someones) has been an utter sack full of cocks in their past. So instead of being a douche about it and telling them how unreasonable they're being a bit of understanding please. Trust me. I've been the douche. Maybe with a bit of understanding they can get over it, and you can make them a much happier person. Or, you know, you could just carry on resenting them until you both hate each other. Whichever.
House is a bad influence on me, and a very good one.