What is going on? Why is he treating me this way?

Anonymous
My boss at work treats me much differently than the other people who work for him. He snaps at me and makes ill-mannered comments especially when he's tired or stressed. He never apologizes when he snaps at me or is weeks late getting back to me about something really important. He also frequently vents AT me. I'm used to people venting TO me when they're worried or stressed about something, and I know he has frequently seen coworkers and others vent to me or discuss with me problems/difficulties in their lives. He puts on fake airs and jokes around with others, so they think he is carefree and feeling good... but with me he doesn't try to hide how he is feeling about things. I am so confused and don't know why he is treating me this way. I am also so worried because he basically controls the rest of my career. I don't know what to do. I came to trust him, and the fact that he is doing all these things is near impossible for me to handle anymore, but I have to continue working there at least for now. He is much older than me... I used to look up to me and have much respect for him before all this started. What can I do? Please help.
What is going on? Why is he treating me this way?
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