I think that at some point you just have to decide that it is just not that important. and there some things about you that are unique, there is no competion for that . you have things to offer that no one else has. the more you let those things shine, the more people are drawn to you, then the more you'll be able to forget that you are not the favorite, cause it just won't matter.
Dont WORRY, we are all second choice,because the first choice is called FANTASY.
You show up as second choice because you haven't hustled to get what you want. If you want something (and it appears you want to be picked) you need to demonstrate to the "chooser" why you should be picked before anyone else.
Have a crush on someone? Show them why you are more attractive (funnier, smarter, hotter) than your rival.
Want a job? Sell yourself as the most qualified and dependable candidate.
Want the lead in play? learn the lines and LET THEM KNOW that you know it cold.
The meek may inherit the earth but those that hustle get what they want.
i can't answer the why part but at least you were chosen ( but I have the same problem about being 2nd choice )