Finding out why they hate you
To find out why people don't like you, hover over the opinion number to find out why: (alas, the cursor isn't showing in this screenshot. I assure you, it's hovering over the number.)
You can find out why people don't like you from any screen that shows their opinion number. Here's another example:
Doing something about them hating you
Vassals often rebel because they don't like you. Once you've worked out why they hate you, try to do something about it. If you have 98% of the titles in a realm, then, as you mentioned in the comments, they probably think your demesne is too large. (Fun fact: "demesne" is actually pronounced like "domain." Language is weird.)
To fix both of your problems (demesne too large, nobles hate you when you give out a title), you need to give out a lot of titles at the same time. Your demesne limit is in the upper-right corner of the screen; for you, it probably says something like "20/5", which means your demesne limit is 5, and you have 20 titles. The "20" will be red, and there's probably a notification at the top of the screen talking about that (unless you made it go away).
You need to do three things:
- Pause the game
- Give away all your excess titles, until you're within your demesne limit.
- Give away some excess duchies, until you're down to 2. (You get -10 opinion against all of your vassals per duchy above 2.)
If you can, arrange it so that you have 2 duchies, and all of the counties within those 2 duchies, and you're within your demesne limit. Spread out the titles a bit, so you don't have one Super Duke who's going to challenge your power.
There's some other advice for preventing revolts over at this other Arquade answer, but this will get you started.
Many thanks to @svick for figuring out the actual problem here.
(As a side note, you probably also have some note about being the "wrong holder type" for your titles. You're a noble, you'll get the best benefit from holding castles (i.e., baron titles) and counties (count/earl titles). Give away your cities and churches to other people. (And when you hit your demesne limit, give away the baron titles and keep the count titles.))