A couple who has been dating for a few years obviously know each other well enough.
Knowing your partner well could mean two things.
You know him / her well enough to know what they like to eat, the colour they like, their personality, their favourite spot on the couch, the whole works lah! So you make them really happy.
You can also make them really, really mad.
Like during a fight or an argument. You know what sends their blood boiling. You know what ticks their anger-meter. And you use it on them. Like the sarcasm he / she hates. Like the silent treatment. Like the roll over and sleep.