Whisper of a thrill

What makes man and women stay together? Besides love, I mean. Could you break up a couple that's been together for like 7 or 8 years, if the best rreason why they are still together is force of habit? Would somebody give up a comfortable relationship, based on something else, for a thrill which could become more? More than they have now? If it was up to me, I would make it so people could onl stay with the ones they love. Even if it would mean breaking up like 75% of the relationships in the world.  I want the brain to be taken out of the ecuation.