He watched her and saw the different emotions that she seemed to be going through.  The songs seemed to be having some sort of affect on her and he just felt this need to make sure that she was going to be ok.  So he continued to watch her as much as he could, while singing with everything in him as if the more he put into his singing the better she might feel. 

If the sky that we look upon
Should tumble and fall
Or the mountain should crumble to the sea
I won't cry, I won't cry
No I won't shed a tear
Just as long as you stand, stand by me