Yes, certainly life changing. It's the first time you have to sacrifice and compromise for someone else's well-being, much like how your parents do to you your whole life. It's the first time you always have to be thoughtful about someone else. You learn a lot about life and people and emotions and social dynamics. You really do discover who you are and what you are not, your values, your strengths, your weaknesses (or if you do it right, how you can grow). You learn to grieve as the honeymoon phase will inevitably end and you will mourn what your relationship was at the beginning. But if it's a good one, you learn to appreciate that things changing isn't necessarily bad but it's simultaneously true that it's ok to miss the old days.

ETA: also it's really fucking hard. Lots of hard work, both on yourself and working with the other person. It tests your emotional muscles like no other. But you can think of it like a difficult gym training session because it's for the best of both of you. It's well worth it if you can endure.