What would make Mars a challenged place for humans to live?
Although Mars is one of the most Earth-like planet in the solar system, living on it would be pretty tough. It is extremely cold on Mars, night time temperature averaging below -100°C. You think the 40 degree weather right now is cold. Try living on Mars. Nobody could survive on Mars without a really advanced winter coat. Another reason that living on Mars would make it hard is the atmosphere. It is like a tiny little needle, though. Since it is very delicate, you would need a spacesuit and some oxygen to bear the low pressure. Your saliva would completely fry your lungs into a nice dinner for possible aliens who might live there. One tiny hole in your spacesuit and you're out forever. Every two Earth years, there is a chance of a dust storm on Mars. Would you like to live on Mars that has a dust storm that lasts for weeks and the lighting drops to 99%? That other 1% is you seeing your death. You might bring some advanced night vision and some sunglasses for the dust. I believe we will have life on Mars soon, with advanced technology and the right people to plan it out.
How may Mars have been different in the past?
Mars was never a cold planet. At once it had a thick atmosphere and known to have a reliable water source on the planet. This would have been a great time to take a visit to Mars and see if we can set human life on the planet, but that was about 4 billion years ago and we didn't have the technology we have nowadays to set a person onto a planet.