What do you hate about designing and building new products?
If you mean dislike, perhaps it's the way in which so many tools have a short lifespan. But with good judgement that risk can be mitigated.
If there was something to be sad about it's that so much good intentioned work is never finished. There are a bunch of ways to mitigate that, though, and the canonical source could be Paul Graham's blog. I'd maybe add Peter Theil's book, but Paul Graham said it first and often says it best in my opinion.