Welcome to the forum, elys.
Sometimes for me, the upset isn't really with the kind person, or how I feel about their kindness. It's often more of a "release" - letting some kind of deep anxiety come out that was held inside because I was in an anxious situation and expected to have to get through it all on my own, or that my needs might be dismissed or mocked.
It happened to me a week or two ago when I went to the bank to sort out some financial problems. I had got so wound up about it, and put it off until the last minute, that when the member of staff turned out to be really kind and helpful, I just broke down in tears.
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When you are fighting an invisible monster, first throw a bucket of paint over it.