Why would two women always be kissing on the lips?

I was with my two friends (we are all women) and my friend Sara asked our friend Lisa for $20 till tomorrow. Lisa looked kinda pissed but handed to her. Then when they thought I wasnt looking Sarah grabbed LIsa as she walked by and pulled her in for a big lip kiss ..like to make her feel better I guess. Thing is she does that to her a lot. She doesnt kiss me like that. hmmmm