A Question.......

Been hanging out with a friends....
One of my friends started talking about his girl who lives like hours away....
He seem to talk about his relationship with the girl, how they talk on the phone.
But my friend is worried sometimes whenever he doesnt talk to the girl for a day....
I asked why?, he just replied," he doesnt know what she is doin or how is she "
Since sometimes the girl doesn't pick up her phone or call him back or reply to the text messages,
he thinks that the girl is just gonna pick up whenever its convenient for her or something or call whenever. He seem to love the girl, and they see each other... But why is he still worried? I couldn't give him an answer since i don't know the girl that well, He seems to always wonder....
But every time they talk, it seem that nothing is wrong with the relationship, its like a normal one.

The question is?

should he worry?
is it just a trust issue?
what does he need to do?
should he just keep wondering ?
should he always keep expecting for the worst?

He kept saying that he always got alot of question in his head...