I'm a bit of a nerdy type and probably labelled as a very smiley, people pleaser.
I've generally found it easier to befriend guys (at school, high school, sixth form, uni, work etc.) but with girls, there's always.. this invisible wall.. (as I've generally been rejected by girls in the past e.g. bullied at school, left out in social groups) it became a self fulfilling prophecy in that I'd never make a huge effort to approach them as I'd always been sorely rejected by them in the past (stuff from childhood really sticks around)
Even when I was uncomfortable talking with girls, it wasn't in a horrible way - if anything, I'd be more of a people pleaser with females, put up with them being unreasonable, etc. more than I would if it was a guy as I felt I needn't to tread carefully and one small bit of me seeming even vaguely hostile might mean I get cut off).
Now I'm mid-20s and have become a bit more comfortable talking with girls at work etc but still, it just feels like a formality on their end - we talk at work (small talk, etc.) and that's it and if anything, it's quite clear that a lot of them are competitive at work so inwardly, aren't genuinely wanting to be proper friends. It's obvious they want to finish the convo and not extend it further (from eye contact etc. or they'll just excuse themselves and go).
With guys, it feels easy, natural (they never look like they wanna get away) and so every social activity I do (holidays, outings etc are always with male friends) which does get me down sometimes like I'm missing something that all other girls seem to have :(