How do the speakers build up the poem's theme?

They usually think of experiences they have had in life.

Explanation:

Poets aren't going to write a poem about something they have never experienced. They always think of experiences that they felt which made them feel the urge to write their poem. If they wrote about the time that they saw a pack of chips fall on the ground and that's it, it doesn't entertain nor make the reader feel any way whatsoever.