Some numbers just don't fit into other numbers perfectly. 3 does not go into 7 perfectly. It goes into 6 perfectly, but not 7.
When you get division problems such as 7 divided by 3, expect to deal with remainders. Just by looking at those two numbers, you can see that there is something wrong. 3 goes into 3, 6, 9, 12, 15, 18, 21, 24, 27, 30, 33, 36, 39, etc, etc. It just isn't in harmony with 7...
It's important to learn your times tables so that you can automatically distinguish numbers that are in harmony and those that aren't.