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