does my best freind trust me?

Me and my friend Sam recently were invited to a party by somone we both new called Dan, at the party there was only Dan's friends with no one else we knew there.

I, myself, am an outgoing person so I joined in with what everyone was doing, but Sam sat uncomfortably in the middle of everyone quietly.

About an hour into the night my friend (Sam) rang his mum up to pick him up. Instead of telling me he was leaving because he was uncomfortable he said his sister was in hospital so his mum was coming to get him.

When I asked him why he lied he said that he thought i would be upset/disapointed in him because he left.

He's my best friend and I would have understood if he felt too uncomfortable since he was out of his element. Why would he ned to lie?