It starts, because you two have something in common. Say a dish, or a philosophy on eating a dish. Maybe you both like girls. Or boys and girls. Or you both care about none of those love and sex stuff. A passion for syntax, torn between lauding and crucifying your boss. You like each other until. Until one gets out of the common ground. Say one found the right love and sex stuff. Shown improved skills in syntax. Became at peace with your boss, liked a dish you barely understood.
You remain friends, because some things remain: proximity to each other and there is fondness still.