Used in sensitive situations to show empathy and support to someone who's down. I know you're upset about the test, but let me cheer you up with some ice cream.
Pro Tip 2/3
Not Mandatory
Even if someone tries to 'cheer up' another, the person may not necessarily feel better immediately. I tried to cheer him up with a funny story, but he still looked sad.
Pro Tip 3/3
Casual Tone
Usually used in informal settings among friends or family, not in formal situations. When Sarah lost her job, her friends came over to cheer her up.