There are a number of angles that need to be looked at when kids don't have friends. Here are just a few of the possibilities:
Some kids don't feel lovable and therefore assume that no one would want to be their friend.
Some kids try to make friends but go about it the wrong way.
Some kids have social anxiety and are afraid to approach other children.
Some kids have had close friendships and lost their friends. They may be afraid to make new friendships in order to avoid more losses.
Depending on the reason for the lack of friendship, that is how we would go about addressing the problem.
For example, one of the ways I've addressed #2 - "Trying to make friends but going about it the wrong way," is by re-creating the posters below together with my students in our sessions, and teaching the reds and greens of friendship behavior.
Each time I work on friendships skills with a student who needs this strategy, I modify this poster and individualize it based on that particular student's needs.
I also create a poster of the red behaviors that are making the student lose friends so that these red behaviors can be addressed. Oftentimes, for each red behavior, we try to make sure to give the student a green alternative.