Wat is Happening in the query?

Wat is Happening in the query?

I am trying to add two values from two columns, But i end up getting few
null as a result, because one column have a few null Here's my query
select COL1,
(COL2)+(COL3) as Total
from Table
Col2 has values say, 1, 2, 3 , 4, 5
Col3 has values say, 5, 3, Null, null, 1
Column Total is spitting out NULL for (3+null) and (4+null), but the other
values are as they should be.
Could someone tell me why this is happening?