Many owners have both Boxer puppies and dogs that lean on them. Depending on your mood, this can be quite amusing or frustrating, as a 40, 50 or 60 pound dog leans against you, placing all of their weight on your side.
A Boxer often presses the whole side of his body into his owner or sometimes the back side. Boxers will do this when sitting or standing, but will purposefully position themselves close enough to their owner so that they can do "the lean".
The first element to point out is that just about every Boxer puppy or dog only does this with humans that they trust. And this says a lot! A Boxer will not lean on neighbors or strangers that you may encounter along the way of a daily walk.
This is a form of communication and it can mean one of several things. In general, when a Boxer leans it is a way of making contact (literally and figuratively).
It can mean: I want to be close to you (showing how much this breed thrives on human contact), I need some attention (showing how the Boxer breed needs praise, pats and cuddling as much as other dogs - even "lap dogs" and it can mean don't leave me (which shows that the a Boxer does prefer the company of his humans as opposed to being alone and some do suffer from Separation Anxiety.
For dogs that have trouble staying home alone, creating a fun, comfortable and interactive area for those times is key to a puppy or dog's happiness.