How do you tell why you lost in Football Manager?

Well there can be multiple reasons for it. I'll just go with most known ones; set up your tactic in a suitable way to your team, -DO CARE ABOUT- scout reports on a specific team when they come and set man marking accordingly. Rotate your squad frequently so your men don't become tired and put less effort. Also, try to look at match stats to see what you're lacking, if you're not good on possession make a training schedule accordingly to it.