TD, if "best self" is a concept you apply only to yourself, why the statement that your hypothetical weight-gaining SO would no longer be his "best self" and wouldn't like himself? How do you know unless you ask him? Since you can't ask him because he's hypothetical, it seems you're making an assumption based on stereotypes of overweight people/couch potatoes.
I admit I've never seen the term "best self" used by anyone else and I personally find it distasteful because it implies some objective standard of quality and seems to mean something very different from making the best of a situation or striving to be the best you can in some particular domain that you value. But you obviously have the right to judge yourself however you like. Applying it to others, which you have done in this thread and others, is what I find problematic and potentially offensive whether you mean it that way or not.
I still do not follow where you are seeing that I am applying anything to others. "Best self" as I use it and have seen it used is, as I continually state, not
objective and is exactly "making the best of a situation or striving to be the best you can in some particular domain
that you value."
In other threads I have said that I want each person to be his or her best self becuase I want people to be happy. In other threads, my comments are in response to someone who is unhappy with something or another. I always say, if you can change it, do. If you don't want to, don't, but then don't complain about it. It doesn't matter to me either way, aside from I would like people to be happy however they get there. If someone complains that she has brown hair and wants blonde, I would say, "then dye your hair blonde." If she says, "I don't want to," I would say, "then be happy with brown hair." It's not a judgment on whether the person should have brown or blonde hair. It's about whether they are happy how they are, and if not, what they will do about it.
Regarding the bolded, no, I am basing this on actual SOs I have had. We have discussed these things. It is important to me to be with someone who takes care of himself and is fit and active, both physically and mentally. It is also important for me to be with someone with whom I am comptable and I am not a couch potato. It really does not matter to me whether anyone else is - I just would not choose them to be my SO. I don't see how this is offensive, unless for some ridiculous reason you want to be my SO!
To clarify, for the hypothetical couch potato morph to happen, my SO would have to have changed fundamental parts of himself. That just seems highly unlikely.