The theme is to tell the readers that things aren't always what they seem. Trysdale was reluctant to accept cactus as a marriage proposal.
When he received the cactus, he kept it aside and didn't try to understand the tag. Trysdale's pride was the main source that lead him to a failed relationship and he was alone in the end.
Trysdale couldn't stay with the woman because he did not know Spanish. He realizes that he was responsible for his sufferings.
