Constantine left because he wasn't interested in Ashley. Maybe most of them stick around even though they're not interested in the bachelor/bachelorette, because they hope to have their own season. Brad had to stick around his entire first season because he signed a contract. I think Ryan announced that he wanted to be he next Bachelor because a producer likely told him to say that... to help the show's ratings. I don't know what the deal is with Ryan: maybe he's just shallow.