I think some of this comes down to the fact that after a long time working you see what look like the same problems appearing and being solved, and then they appear again and a new solution comes along, and then again and again...

It may partly be a failure of pattern recognition, where the problem being solved seems to be similar to what has been done before, but actually it is different enough that a new solution is warranted.

Or it may be frustration at the fact that the problem was never really solved in the first place.

Or it could be you look at a solution and think, this is just like a previous solution with some minor changes, nothing new to learn.