Guy's Behavior

What does it mean when a guy has extremely unstable moods?

There’s a guy, an acquaince/friend of a friend, who really puzzles me. He is the guy with the most unstable moods I ever met. One moment he is the heart of the party, dancing, being loud, trying to motivate people to party even when everyone else is quiet and not really into it. A second later he gets quiet and aloof, for example at a dinner with friends he will just go sit away from everyone else and stop interacting and then he’ll say he’s bored or tired and will go home early. He is outgoing and seemingly at ease with people, but than maybe he spends the night biting his nails without realizing it and sits with crossed legs nervously moving his foot. And the switch from one of these versions of him to the next is sudden and seemingly unprovoked, in one evening he usually goes from one to the other

What does it mean when a guy has extremely unstable moods?
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