Day traders must control their emotions. The slightest lapse in controlling your emotions will ruin your accumulated hard-earned profits overnight.
For instance, after having several losses in a trading session, you are afraid of having a losing day. This fear spurs you to over-leverage and blow up your trading account with a single trade.
How many times have you blown up your trading account because you lost control of your emotions?
Furthermore, having your emotions on a roller coaster everyday is not sustainable.