Henderson was 
reluctant at first

According to Roy MacSkimming's incredible book Cold War, Paul Henderson considered not attending Team Canada training camp because of prior arrangements.
According to the book, Henderson and his wife Eleanor, had planned to be in Germany vacationing along the fabulous Rhine River in late August. The invitation to Team Canada's camp interfered with those plans. 
Henderson was initially reluctant to accept coach Harry Sinden's invitation. However his mind was changed when tournament organizer and Henderson's agent Alan Eagleson convinced him to go. Olympic shooter Susan Nattrass also convinced him to go after telling him stories of how great it was to represent Canada in international competition.
Henderson wasn't the only one who was reluctant to come. In fact 4 other players did turn down their invites.
Rangers goalie Eddie Giacomin was still recovering from knee injuries, so that is acceptable. Montreal defenseman Jacques Laperriere's wife was expecting a baby, so he stayed home with his wife. Kudos to him.
Bruins defenseman Dallas Smith turned down the invited because he had to harvest the crop on his Manitoba farm. That might sound a bit odd nowadays, but don't forget back in 1972 players weren't making ridiculous amounts of money like today. Smith likely couldn't have paid for help with the farm while he was off with Team Canada.
Finally there was Walt Tkaczuk, the great Rangers centerman. He turned down invite so he could instruct at his hockey school.  That was very noble of him - keeping hockey at the grassroots level and teaching the kids.
After all was said done, you have to wonder if Tkaczuk ever regretted not taking the opportunity. Somehow it is unlikely that Henderson ever regretted putting off his German vacation off until a later date.