There definitely is a balance to be had in communication, and that balance will be different for different relations. I would recommend a double-take on your use of the word partner, though.
I would agree with what you say, if referring to a relationship, but the word partners is a stronger dynamic. Being partners, can mean different things to different people, but I think the problem is that we are conflating a romantic partnership with a relationship. Your statements ring true for the latter, but not for the former.
A partnership, requires that you deeply consider how your behaviour impacts your partner, because you are fundamentally ‘in it together;’ that’s what distinguishes a partnership from a relationship. You still have your independence, but it’s a part of another whole. The success of the whole takes precedence, because it can only succeed with the well-being and success of the parts. It differs from harmful forms of co-dependency, because you can’t forge a good partnership by only giving. Partnership is aspirational and challenging, and has it’s own issues to navigate down the road, and frankly isn’t right for everyone.
While you may have emotions and want to keep them to yourself, you have to consider how that helps the partnership, or whether it actually does. In a relationship If your behaviour reflects your emotional turmoil, that will have an impact on your partner, and that will make them uncomfortable. Of course you want your partner to feel amazing, and when they don’t, you want to help. While it’s no reason to break down or feel worse, not communicating is affecting the other part of the pair, so now both individuals feel worse and the partnership suffers. But the whole point of a partnership is to work together to maintain something beyond the capability of the individuals.
To that end, if you are truly trying to maintain a partnership, and not just a relationship, then they aren’t your thoughts and emotions anymore, not solely. That’s an, and maybe the important distinction.
From a practical perspective, what reasons are there not to share with a partner? If it’s serious, why not get their emotional support? And why not prime them to understand what’s going on? If they have a sense of the issue, they may be better-able to support you, whether that’s by giving space or by avoiding a certain topic, for example. If it’s not serious, then why are you letting it impact the partnership negatively? If you only get about 30,000 days on this planet, why lose one (or more) to something trivial? And, if it’s a matter of needing time to figure out how to explain it, you can always ask for a day, and talk about it later.
What you identified as selfish motivations, are indeed selfish, but not necessarily wrong or counterproductive to the relationship. How the situation is handled matters a great deal; for example, learning to listen first, and only offering help if solicited is challenging but important. Learning that you don’t need to know everything is important. This, to me, is the most important part of your post. Being able to provide good support to someone under emotional duress is critical. It will improve a relationship, and is necessary for a partnership.
To wrap this up, all of what you said is reasonable and true for a relationship, whether it is a week old, or has been going on for 30 years. But the word partner just doesn’t fit in there, because it implies something different. A relationship is a link between two individuals, a partnership is a new entity that encompasses them. It’s important because, the relationships that I’ve seen that last the longest, and happiest, are frequently partnerships. They aren’t for everyone, are harder to build and maintain, and also harder to enter (fewer people are partner material than are relationship material)