Think of fractions as a circle each circle is the same size. So the 1/2 is a circle divided cut in half, the 1/3 is the same sized circle cut into three parts. Now think about it, the pieces of both the circles will be of different sizes right? the pieces in the 1/2 will be bigger and the pieces in 1/3 is smaller. Can you really just add 1+1 there? Nope, that's like adding apples and oranges. So, to add them together, you have to ensure all the pieces are the same so you can add them properly and so it would make sense. Which is where you find the LCD of the fraction to ensure the pieces are equal and you can add properly.