Why did he kiss me on the lips?

Anonymous
My best friend of 27 years is a guy. We've always greeted each other with a kiss on the cheek and hug. The last time we were together hanging out, his kissed my lips instead (mind you I'm not complaining). Why all of a sudden would this change? I don't know how to interpret this gesture. Any help would be great!
Updates
+1 y
this occurred three times over the course of the visit. if this helps with your advice.
Why did he kiss me on the lips?
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