彼は楽しみで我を忘れている。

he; him; his; boyfriend

topic marker particle; indicates contrast with another option (stated or unstated); adds emphasis

enjoyment; pleasure; diversion; amusement; hobby; anticipation; looking forward to

indicates location of action; at; in; indicates time of action; indicates means of action; cause of effect; by; and then; so; indicates continuing action; indicates certainty, emphasis, etc.

obstinacy; atman; the self; the ego

indicates direct object of action; indicates subject of causative expression; indicates an area traversed; indicates time (period) over which action takes place; indicates point of departure or separation of action; indicates object of desire, like, hate, etc.

to forget; to leave carelessly; to be forgetful of; to forget about; to forget (an article)