How can we call Haazinu a Shira when there is so much punishment in it?

The answer is that punishment is not something that Hashem does to take revenge on us, it is something that Hashem brings on us because it is good for us.  We understand that everything that Hashem does is for our ultimate good.  When we stand in front of Hashem before Rosh Hashanah we may feel discouraged when we think of the ways that we didn't do that we promised we would last year.  It is tempting to fall into despair and to feel that there is no use in even trying. We have to know that even when we are deserving of punishment, even though we seem to push Hashem away, he is always there.  In an effort to direct us in the right direction, Hashem will send nisyonos our way, but it is not to push us away, it is to bring us closer.  

from the Shiur 

Parshat Nitzavim & Vayelech: Symbiotic Song Teacher: Mrs. Shira Smiles

Lately I have spoken to many people who are all bordering on despair.  They feel that their lives have become so difficult that they do not know how they are going to keep going.  I feel that this answer is a very powerful insight to help us to get past this feeling.