Me as a 5-year-old: How do you relate to the Twin Towers?
Friend: What?
Me: Every time I think of them, I feel sad.
Me as a 5-year-old: How do you relate to the Twin Towers?
Friend: What?
Me: Every time I think of them, I feel sad.
OK, listen up, buttercup. The joke is that the person is 'relating' to the Twin Towers by feeling sad when they think about them, referencing the 9/11 terrorist attack that destroyed the Twin Towers and killed a lot of people. It is funny because you are stupid and because it's dark.