Because it was overly long, way too hard, boring, asked to pay attention to way too many factors to get it done, other?

What side-quest was it and for what game and what made you hate it?

GTA 5 in general.

Never found all the tri force pieces in the original Windwaker though I guess that is an integral part of the game not a sidequest.

best side quests i found where from gothic 2 (e.g. thieves guild quests)

Dragon Age Inqusition had an odd amount of collectible side quests. Those skull things and things you pick up? Pointless as the treasure you get at the end, isn't even good (you can create better). So... I just don't play the game with them. Ignoring that massive side quest and the game because instantly better (I loved the game but wasting time looking for things like that is just time consumingly silly).

EDIT: Point mentioned below.

Dragon Age Inqusition was so tedious

Devs love thos, most of the time they feel as a cheap way of adding more hours of "gameplay" to the game.

Collecting items in xenoblade.

Pretty much anything in WoW... Fetch quests are annoying, never liked them one bit. The worst part is great games that suddenly offer a shit quest.

There was this quest in WoW where you had to slaughter these snake-people for their scales.  I killed dozens of those bastards before I had enough.  I had to actually wait for them to respawn a few times.  Sometimes random drop chance can be an absolute pain.