Now, being optimistic is one thing but being greedy is not a wise thing to do. Why ? I'm glad you asked.
See, if you short USDCAD and long Oil at the same time and if Mr Market decides to play you out, what are you going to do ? You suffered double losses in doubly quick time.
That means you will be game over sooner than you expected. And that is how portfolio get burned over and over again. Just because you won the last few rounds does not make the next trade a sure winner.
That is why we have stop loss, we have frequency of trades and avoiding concentration of risks too much.
Again, we want to play safe and win consistently and not have too much in our trades.