What does Brett live for?--motivation

What is Brett's motivation? I often wondered that for why she goes through men the way she does. She obviously is in love with Jake and yet she won't stay with him. What does she gain from this. I feel as though she is here for the ride of love. And when the ride begins to go towards something too serious she jumps out before the man even notices it. The men usually begin to show their love for her in some way. For instance, on page 220 Jake talks of "...he loved Brett. Everything of which he could control the locality he did in front of her all that afternoon." This is Jake speaking of Romero. But we know this cannot last. I think Brett is in love with the idea of love.