Thank you for the opportunity to help you with your question!
This is true because if you increase the number 1 by 1 the result is 2.
If you look at a number line and start at 1 and then add 1 it is equivalent to moving right on the number line one space to the number 2. This completes the operation of addition and is the solution to the problem.
Please let me know if you need any clarification. I'm always happy to answer your questions.