Will letting your dog sleep with you in the bed really cause aggression?
No, your normally submissive dog will
not suddenly turn into a dominant monster if you let him wiggle onto the bed at night. Nor will you be raising your young puppy up to be an aggressive monster. It is true, however, that allowing your dog a position of authority can exacerbate an already existing dominance or aggression problem, and just as true that removing that one privilege will not solve your problem (and if it's not a problem yet, it will be).
When it's going to be a problem:
When he's taking over the pillow and there's no room for your head. Seriously, if he's slowly started to take over the prime soft spot in the bed, and you're left grasping the very edges of the pillows while he stretches out in luxury, it's time to demote him back to the foot of the bed, or off the bed altogether.
If he races you to the bed, settling himself
first, before you get there. There's no problem if he's been snoozing in your bed and hops off as soon as you show up, but another altogether if you need to order him out of the way, or physically haul him off so you can lay down. If he's challenging you for authority, you need to put a stop to it immediately, and make him
work for everything he wants or needs, and reassert yourself as the pack leader.
Your dog will need to be demoted to the floor, or possibly even removed from your bedroom altogether. Remember that the bedroom, and especially the bed, are the most comfortable and desirable locations in the house, and belong to the leader. your dog's presence is a privilege, and he must not forget that.