Well, no. It doesn't have to always be that way. Some people prefer to top, others prefer to bottom, and some just don't care. And although it's possible that both parties can switch if they like each other that much in that way, not everyone has to yknow. And people come in different shapes and forms. Not all bottoms just take, and not all tops give. One of my personal fav troupes is a dominant bottom, as they tend to initiate more, and submissive tops. But like I said, don't be too conscious or sensitive about the positions. Both parties could just prefer to have their needs satisfied as ordinary bottoms & tops. Not everyone wants to switch.
I hate why most BL authors don't like to make their characters switch positions. and always uke depicted the recipient of love and seme the giver of love. I mean, when two people like each other there must be a mutual give from both parties and not just give and take, not because someone seme he should love more. *sigh*