So 1 + 1 = 10, because in binary when u have a number bigger than 1, you carry over 1 and put a 0 in place. That's why when you add 1 to 1 in binary, you carry over a number and put 0 in that place. That's why 1 + 1 in binary = 10 in decimal. You can also think of it this way:
1+1=2, 10 is 2 in binary. Therefore, 1+1=10.
I apologise if your brain hurts.