What is the glue that forms the bonds of friendship?

Supports our social identity
Has a similar sense of humour
Shares personal information about her/himself
Listens to our issues and personal information
So friends need to communicate regularly
Might not always agree with us, but is non  judgemental in their responses
Probably shares some interests or passions
Feels loyal and trustworthy

What has made you a strong bond with your best friend?