I've always been like that, I am only bothered by uncompleted stuff.
How do you motivate yourself to complete the games you didn't enjoy?
That's easy, I don't play what I won't enjoy.
When did you start this completion trend? From the start of your account or did you have to overcome a huge backlog to achieve this?
See first answer.
Do you feel like 100% completion is an overall better experience than a normal playthrough (where you don't mind the trophies)?
Usually a normal playthrough precedes a competition playthrough, so I get both and I don't "prefer" either, I just enjoy them for what they are.