It really all depends on if one is "happy" with the way they are.
Sometimes if a person suggests their mate lose weight it's really a confirmation of what he/she may already have been thinking before they heard their suggestion. If someone is overweight and they know it as well as not being happy about it then it's not exactly a shock to hear a mate suggest they make an effort to lose weight. It's not so much what is said as much as how it was said that counts.
Not long ago an obese woman was told by her husband that he was afraid she was going to die and he did not want to lose her. She had become diabetic and had very high blood pressure. He told her he did not think he could go on without her. She went on to lose over 100lbs.
The "knee jerk" reaction is to become defensive or let our ego run wild whenever we are criticized about anything especially if it's of a (personal) nature. If you're "in love" their opinion matters to you.
Nevertheless if in your heart you actually (agree) with them but your feelings are hurt because they "confirmed" what you were already thinking it makes no sense to attack them for their honest opinion.
Life is a personal journey. If (you're happy) with the way you look and your mate is not then it just means he/she is not "the one" for you.