Tell me the forbidden facts
The atmosphere on Mars is so thin because its core is solid, meaning it can't spin and create a magnetic field like ours can. This means the solar wind is stripping the atmosphere away! The MAVEN misssion determined that 100 grams of atmosphere is stripped every second assuming local space weather is calm (i.e. no solar storms). This means any future colonists will need to be deliberately wasteful and create as much greenhouse gas as possible to thicken the atmosphere and warm it up!
Another thing on Mars is the soil is, for the most part, PERFECT for sustaining life. We'd need to seed it somewhat with bacteria and fungi, but otherwise, plants can more or less grow in it no problem. The only real barrier is perchlorates, which are toxic. But... since Mars seems one of the most likely places in our solar system to house life besides Earth, perchlorate could play a role in sustaining some bacteria-like life forms. The chemical reduces the freezing point of water allowing liquid water to exist on the otherwise freezing cold planet. Earthbound bacteria also can use the chemical in place of oxygen to help with metabolism! It creates liquid water and oxygen, helping colonists out.