i could on for decades about everything wrong with BL but here are a few things that come to mind:

-All the miscommunication at the start of season 3 with the FWB stuff
-Brooke cheating on Lucas with Chris
-Lucas messing with Rachel
-Lucas never understanding Brooke and not knowing enough about her to submit her designs
-Brookes constant sadness and insecurity when she was dating Lucas
-Brooke's constant jealousy of LP
-Brooke throwing a party for Lucas after Keith died. Absolutely not what Lucas wanted.
-Lucas not calling her when he was gone.
-Lucas cheating on Brooke with Peyton
-Lucas using Brooke for sex basically in season 1
-Brooke being a horrible influence on Lucas in season 1 and his relationship with Keith getting rattled because of it
-BL not even being friends when they werent together
-Lucas going on a road trip with P right after he and Brooke got together.
-Lucas using Peytons words from the same letter, and then Brooke overreacting.

I could keep going but ill stop.

Just in general Brooke was the most insecure and unhappy when she was dating Lucas and even she knew this. I just dont really get how people can love them when nearly every episode there was some problem. They just didnt work.

Jake/Peyton were fine, nothing was wrong with that relationship imo except for one major thing: Peyton loved Lucas. lol.