I say "some people" because I'm aware that for lots of people sexuality is set in stone - they're either gay, straight or bisexual (or whatever) and that's all there is to it and nothing will change that.
However I do think that for some people it is more complex than that and their sexuality does change throughout their life. I'm pretty sure my sexuality has changed at various points in my life. I say "pretty sure" because I'm still trying to work out how that would actually work.
I have heard of other people having a change of heart throughout their lives (mostly women, but also a few men) and their sexual preferences changing at different points in their lives so I do think that sometimes it's more complex than people just being born that way.
I've also heard the theory that people who are fluid with their sexuality are actually bisexual therefore it's easier to be fluid and be attracted to either gender at various points in their life IYSWIM. But I'm not sure if I would label myself bisexual, I just really think it changed.