Perfect Chemistry by Simone Elkeles / endings

Perfect Chemistry is the first novel I've read by Simone Elkeles - looking forward to reading more.  I did have a little trouble believing Brittany and Alex would have a long-term relationship.  So I guess the epilogue didn't work for me.  Sweet, but not very real.  Do readers want "most likely"outcomes?  Do they want the "ideal, but not likely"? That Brittany and Alex would date through their first years in college works for me, but I would think they would grow apart.  Marry other people and stay friends?