What an exciting question! I think there are many reasons why humans are trying to find life on Mars. However, the main reason seems to be the large fascination that life may exist beyond Earth and that some other living beings are somewhere out in the vast universe.

Mars is the closest planet that could potentially have some form of life similar to that on Earth and would be the most accessible place in our solar system, galaxy, and universe to explore. So I think it really comes down to a curiosity about life beyond our planet and the first place to look would be the most similar and closest place to Earth being Mars.