Gus is 9 months old and doing the same thing! I went thru this with Izzy too, but she was a little older, maybe like a year old or 18 months at the time. She is 2 years old now and doesn't do it anymore. I personally think it's a dominance thing... your dog is trying to dominate you !!! By lifting them up and you doing what THEY want, they are in charge, and you are the follower. Izzy always jumped into the car and then one day decided that she was just going to put her front paws in and have me lift her. I fell for it a few times, but after I figured out what it was all about, I would stand there until she jumped in. So now, with Gus, I am trying to just ignore him when he puts his big head on my bed, or front legs, wanting to get up and acting like he can't jump. He is getting better jumping into the car. I know he CAN jump because when we are outside playing he can jump up and his eyes are at eye level with mine, and I am 5'10'' tall!!! So, I know he has no problem jumping. If your boxer can jump other places, they can jump on the bed or in the car too!!! I think, especially in their "teenage" years, they are testing you more than ever. So, you have to let them know that YOU are in charge, and not THEM, or this could create problems down the road if they think they are the leader of the "pack". Just ignore them when they want help jumping, as hard as that is. I know it's very hard, cause they are just so darn cute and they look at you so pathetically like they can't jump. But don't fall for it!!!
Good Luck to everyone!!!