I admit that I'm not one of those kind of people who has 'always' wanted to do psychology. I'm just one of those unfortunates who's pretty good at most academic subjects but isn't very good at anything, so no particular subject has ever stood out to me as the obvious choice. I've always loved physics but really wasn't good enough to do it at uni.
I chose psychology at A-level because I figured I'd probably be interested in it and I was correct. I'm also pretty good at it and I like the combination of maths and essay skills needed. There are areas of psychology I'm particularly interested in, especially the biological and stats sides of things, but I'm not going to lie and say, 'oh, I LOVE psychology, it's like my DREAM subject and it's perfect in every way!' It just was the subject that made the most sense to study, and it still is, I hope.
I might be saying something completely different in three years' time, though.