Well there's the physical intimacy, and there are those little things that just set the mood, like a peck on the cheek and a small compliment to make her day, or a neck rub in front of the TV to make his day.
Of course it's important to get along too. If they spend their time fighting, then there's really not any "best things" to discuss about it.
Psychologically speaking, some people think they like being in a romantic relationship, when in reality they only like the IDEA of being in a romantic relationship. They like the IDEA of having a partner to share intimate moments with, so they find someone to turn that into a reality. I know this from personal experience from my teenage years.