Somehow I developed a bad habit long ago: it became important to me to not let anyone know what was going on in my head. Too much risk of being laughed at or rejected, I think. And a general free-floating fear of What Might Happen, without any specific bad scenarios in mind. And a deep-seated gut feeling that it's important to maintain distance, not let anyone too close, the whole "I am a rock" thing.
(And that's one reason I like email and journaling; both provide ways for me to feel a little safer in saying what I'm thinking/feeling. Talking to people is hard, and scary; writing to people gives me just enough distance to take the edge of the scariness. I can certainly understand that for some people it gives too much distance, doesn't feel like human contact (and I certainly know that the lack of cues like tone can be a problem), but for me sometimes I need a little distance before I feel safe talking about what's important to me.)
So one unfortunate pattern I too often find myself in is sitting with a group of people, and seeing someone I particularly like (or am particularly interested in) arrive, and wanting them to come sit by me but not wanting anyone (including the person in question) to know that's what I want. And then if the person does come sit by me, I make the worse mistake of acting nonchalant, like I really don't care that they came to sit by me; I sometimes even act like I'm ignoring them, and talk to someone else. (This is especially true in a group where I'm uncomfortable with the rest of the group knowing I'm interested in the person.) Which sometimes leads to the person getting up and going elsewhere. Which makes me jealous and grumpy, and I start to kick myself mentally for taking too long to show interest.
(Of course, this is all complicated by the fact that sometimes when I don't pay attention to someone who's sat down near me, it's because I don't know them or don't feel like I have anything to talk with them about or am just feeling twitchy about social interaction. If I ignore someone, it doesn't necessarily mean I'm interested in them.)
I suspect that more than one person reading this will think they know exactly who and what I'm talking about, so I should note that I'm not talking about any one person; I do this unfortunately often. (I've probably done something similar with three or four different people in the past couple weeks.) For example, at Christmas dinner at the Evanses' last year, I managed to sit next to Lola, and then we spent most of the meal turned away from each other talking to the people on our other sides, even though I was acutely aware that she was sitting right there and I wasn't talking with her. Later, through judicious application of email and telephone, we worked it out. But that's not always feasible. And anyway, even when it does work out in the end, I get frustrated with myself for falling into the same old patterns that have been not serving me well for twenty years or so.
This may just be a subcategory of my general feeling that my social interaction patterns are, in too many ways, still stuck in middle school. Shyness around people I'm attracted to, distrust of popularity (and simultaneous desire for it), head games, cliques, wanting to please authority (and simultaneous distrust of it). I try hard to subvert those patterns, to notice when I'm engaging in them and stop myself and try to live up to my own expectation of maturity and adult approaches to things, but sometimes all the old insecurities and fears and unfortunate behaviors still bubble up with little provocation.