is it bad??

I am not sure what would be offensive about this statement; you will need to explain more about the context and how you thought it was offensive.

If you are the girlfriend:
Can you tell us how old you and your boyfriend are, how long you have been dating, and what the circumstances were? Were you two friends before dating?

I can only assume that you were thinking your boyfriend was always in love with you, and he should not have had to "grow" into loving you? Mature love is usually a type of love including love that "grows" from one that is (typically) based on physical attraction and lust into a love that is based on mutual interests, goals, respect and attraction both about physical and "inner" beauty. This is said without knowing the context, non-verbal cues and other important information that only the receiver would be privy to.

If this is not you:
This question is better discussed with close friends, as we can not accurately provide medical or health information based on a hypothetical situation.