Answer:
He thinks of the scene described in the poem.
Step-by-step explanation:
To be pensive is to be "engaged in, involving, or reflecting deep or serious thought". The speaker is on his couch lost in thoughts
For oft, when on my couch I lie
In vacant or in pensive mood,
That way, he can forget about the rest, since what he thinks about will eventually make him happy:
A poet could not but be gay,
In such a jocund company