The humor. Simple as that. When a comic strip has a really funny situation like a character constantly getting into silly misunderstandings or doing things that are completely unexpected. For example, a superhero who is afraid of spiders and runs away screaming when he sees one during a fight with a villain.
Well, the characters play a big role. If they have unique personalities that are easy to relate to or are just really over - the - top. In a comic strip funny story, you might have a lazy cat that always tries to avoid doing any work but ends up in more trouble because of it. Their expressions also add to the appeal. A character with a really goofy face can make a simple situation seem really funny.