You marry to have a family and build a legacy. When you marry to just have hallaal sex, or because you 'love' your partner, or any other emotional reason, eventually those feelings will dry up, then what? No one is happy all the time just as no one is angry all the time. With that logic, no one is in 'love' all the time either and if a couple can't figure out why they are together beyond the emotional feelings we experience day to day, you get divorce.