I would never change myself for anyone. Sure, other people have encouraged me to improve certain aspects (such as being less passive, getting some form of exercise, and being less of a slob), but I only change when I want to and am ready for it.
That's probably the biggest reason I hate "Grease" (aside from not liking most of the characters). The girl doesn't get the guy until she changes who she is. That's a horrible message.
Other people will come and go. You are the only person who'll be there for yourself constantly, for the rest of your life. Stay true to yourself. Plus, if someone doesn't like you until you change, they don't really like you for who you are, and they're not worth it.