A solar flare or a gamma ray burst might knock out satellites and electrical equipment and damage earth's atmosphere. In the worst case, they could kill a few billion people. But Earth will still be more hospitable than Mars.
>Same reason people crossed the Atlantic, to take part in something greater than themselves.
People crossed the Atlantic because they wanted gold, or slaves, or timber, or land, or because they wanted to escape from religious persecution, or because they were slaves or prisoners transported against their will (by someone who wanted gold, or timber, or land). They received resupply, not as charity, but from companies which expected to receive gold, tobacco, timber, or other goods in return from their investment.
Colonization is driven by economics. That's why thousands live permanently in Norilsk, Yellowknife, La Rinconada,and Kalgoorlie, but nobody lives permanently at Amundsen-Scott, under the sea, or in LEO. If history is any guide, "how will this turn a profit" is the question any Mars colony needs to address if it is to have any hope of prospering.