Psychology of Trading.
Demo, there is no real money and so trading with any amount and high leverage doesn't cost anything even when you lose.
However, in the live account, there is real money at stake that you're risking, your aim is to win and not lose on any trade. There is the emotional commitment that may alter your trading plan since real money is on line and the focus is to make profits. Remember you may make profitable results trading a demo account, but the results in a live account often differ because you are under pressure to not lose real money.
Notably, some brokers have manipulated their software such that in Demo accounts the customer looks good by making profits to create confidence to move into live accounts and then rip them off in real accounts, considering they have not mastered the art of trading.