When you switch seats with someone, why does the seat almost always feel warm?

According to the Second Law of Thermodynamics, heat always passes from a warmer item to a cooler one. Heat passes from the previous sitter's warm bottom to the cooler seat. When the next sitter sits in that seat, he or she is feeling the warmth that passed from the previous sitter's bottom. Of course, the seat the next sitter was originally sitting in did not feel so warm because, as it was already cooler than that person's bottom, heat from that person's bottom was passing into that cooler seat. Instead of feeling warm, that seat was absorbing heat away from the sitter's bottom. Eventually and gradually, that seat will no longer feel so cold.

I base my answer on my own understanding of The Third Law of Thermodynamics; however, one can read about such law on Widipedia.

Thank you for warming us with your bottom.

https://en.wikipedia.org/wiki/Thermodynamics