Since it is hard to get back to Earth from Mars, people, who serve lifetime sentences should be sent to Mars to repay a debt to society.
We are not made to live on other planets, we should stop trying.
Before colonizing any other planets, we must take care of this one. Going to Mars is ridiculously expensive, this money is better used to fight climate change.
Colonizing space goes against some religions.
If Mars is ever colonized, who does it belong to (e.g. scientific community, union of countries, whoever gets first)? Why?
A human on Mars will inevitably contaminate the environment with their biological material. We are better off sending just robots.