Guys, Why will a guy friend suddenly not want me to see the photo of him and his girlfriend?

It's not like I've not seen it before, nor do I not know he has a girlfriend. Why will he suddenly tilt his phone away from me when his phone light up and showed the photo of him and his girlfriend?

(There were no other messages or notifications appearing at the same time)

Guys, Why will a guy friend suddenly not want me to see the photo of him and his girlfriend?
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