I like C.S. Lewis's description of "falling in love" as a sort of milk-and-water phenomenon that is a mixture of desire, vulnerability and charity. I'm pretty sure that's somewhere close to how he described it in "The ScrewTape Letters".
Charity is needed for love to be sustained. The newness wears off. Little annoyances that were easily overlooked in the heat of desire and emotion can seem magnified without the former. The perception that the person is one's completion or perfect-for-you doesn't last the test of time. All relationships have trials. People who understand these things and are still "in love" make an effort to practice charity, and put off those things that hinder or destroy it.