whining while hiking

So I just took my almost 2 year old labradoodle on his first hike.  He is very sociable, playful, friendly, etc.  I have always been very regular with walks twice a day and playtime at the park, etc.  He's a very happy puppy.  However, he still pulls when other dogs or other people walk by.  So I decided to get him a backpack for our hike because I heard this give them something to focus on while on hikes to help with pulling, etc.  It was not heavy, I put a water bottle in each pouch and some treats.  He seemed fine walking around with it.  But on the hike, he whined consistently.  We were with a friend and her dog and it was obvious the backpack was not too heavy because my dog had no problem trying to keep up but whined the entire hike.  Although happy (major tailwagging), people stopping to take his picture and petting him, he was obviously very happy.  So why was he so whiny?  

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