This is perhaps the easiest thing to lose without noticing.
You stop doing the things you enjoy. You see your friends less. Your routine starts revolving entirely around your partner. Slowly, their approval begins to influence almost every decision.
None of this usually happens overnight. It can happen through a series of tiny compromises that seem harmless at the time.
Being committed to someone does not mean becoming a different person. You can love someone and still have your own interests, opinions, friendships and ambitions.