I think it is interesting that some notions in this thread boil down to the idea that this ain't real math like, say, real analysis.
In my limited view, many proofs are easier. Especially easy ones that are (if that makes sense) more sequential in the required thinking.
I think there is a lot of cleverness in these integral calculations, a lot of creativity to, as you say, come up with some seemingly nonsensical transformation that down the road turns out useful.
Perhaps it's because I am so bad at it, but I find people who does this easily very impressive.
I feel the same way for differential equations, where, incidentally, I also suck.
I wonder whether anyone ever did a study on the cognitive skill "dimensions" in maths.