A relationship is not a thing. It's a shorthand for 'the set of interactions you have with a person'. But people often treat it like a thing. They do things 'for the relationship'. One could interpret this as doing some stuff in order to make other stuff better/longer/whatever, but I don't think that's how people think about it. They don't think about how or even whether the other interactions are affected.
If one is rational and has relationships to the extent one has shared interests with someone, that means the more you change your interests, the less you will have reason to interact.
This applies to romantic relationships, too. If you think of relationships as things, growth in different directions is a threat to the relationship. What this actually means is that if you grow in different directions (if you lose interest in stuff you used to do together), you will have less in common, so less stuff to base your interactions on, so you will interact less.
Because of this misconception about relationships being things, some people fear the loss of the relationship. But this is nothing to fear. Either the interactions will be good and they will continue, or they won't be and then it will be good to stop, and spend your time doing something better instead.
Proposed Translation
'doing something for the good of the relationship' means re-distributing the gains from trade between the two of you so the relationship is a net gain for both of you.
Also
I agree with the post, I think, provided you mean that it is only the loss of a relationship because of differing interests that we do not need to fear- loss of a relationship for other reasons could still be bad.
Wait; what if you like interacting with someone, but need common interests to interact? In this case, growing apart could be a net loss even if the changes in interests in them selves were beneficial.
What do you mean by 're-distributing the net gains from the trade'?
And right: loss of interaction because you don't have stuff to interact about is what I was talking about. Loss of interaction for other reasons -- e.g. maybe you misinterpret the other person as wanting to cause you harm -- may well be bad.
If you like interacting with someone, there's a reason you like interacting with them. Maybe you like the way they look at certain issues, or they're funny, or whatever. This is a type of shared interest: you are interested in their perspective, or humour or whatever. So, you'd interact with them based on that. You might ask them what they think about stuff, or go to comedy clubs with them, or whatever.