One side of the argument is that microgravity will an kill people before they get to Mars.Also if they survive there bones will be shriveled so when they get to Mars they'll be useless
Another reason is that when they get to Mars the tempatures will freeze them to death.It can be as cold as -200 degrees.also if they survive they can get dieseases from the cold.
I understand some people may say we can find new life forms.People will say we can start a colonization on Mars,But many will die in the process if we succeed our mission to Mars
In conclusion those are my reasons we shouldn't send people to Mars to start a colonization.microgravity will shrivel your bones. Also Mars can have freezing temperatures such as 200- degrees