I did a quick google search, and apparently the expression used (我慢は体に毒) is a Japanese saying. Something along the lines of being patient can mean putting others before you, even when it doesn't benefit them. So being too patient can be unhealthy because you'll never be putting yourself first in any situation, and in the cases where the other party doesn't feel any better from your consideration, literally no side becomes happier.
(I only partially looked at the first link, so take this with a grain of salt.)
In other words, it's like Aristotle's golden mean, everything in moderation (but specifically about patience in particular).
The more you know~