I think I upvote mostly like this (I’ll edit this answer if I remember more reasons):
Strong upvote: correct (if applicable) and important.
Upvote: correct (if applicable) and slightly important, or not completely correct but interesting/potentially important (In this case I usually also reply). I also tend to upvote comments under my own posts much more because I feel the need to thank the person who took the time to write the comment somehow.
I don’t always follow these rules exactly, but mostly. For example, sometimes I upvote to close the gap between two comments I consider equally important under a post.
I think I upvote mostly like this (I’ll edit this answer if I remember more reasons):
Strong upvote: correct (if applicable) and important.
Upvote: correct (if applicable) and slightly important, or not completely correct but interesting/potentially important (In this case I usually also reply). I also tend to upvote comments under my own posts much more because I feel the need to thank the person who took the time to write the comment somehow.
I don’t always follow these rules exactly, but mostly. For example, sometimes I upvote to close the gap between two comments I consider equally important under a post.