Attachment theory is based on developmental psychology, first introduced by Freud, and then followed on by Bowlby and Ainsworth.
Basically, when a child has a good attachment with their primary caregiver, they are happier and more trusting. They know that their caregiver loves them and will come back if they leave.
Unfortunately, most people do not understand attachment theory. It doesn't mean that you carry baby around all the time, though wearing baby in the early months certainly has its plusses. It's all about psychological development. Trust and mistrust. And it's based upon your actions as a parent and how you respond (or don't) to your child's needs.
You might want to read up on it a little more, because done CORRECTLY, ensuring a good attachment is exactly what you want for your baby.
Regarding "wearing baby," I also suggest you watch the documentary "BABIES." I saw it on Netflix. Some cultures "wear" their babies like you do...and then when they start moving around more, they pretty much let the kids spend most of their time together. They crawl and walk SO much faster! Parents also let their babies explore and learn more on their own, causing them to be braver and more secure in their environment. The amazing contrast is that the documentary also shows those cultures who basically sanitize everything for baby and carry baby less. Those babies develop physically and mentally SO much slower. It's incredible. So your "friend" is dead wrong.
I think attachment parenting, like so many other types of parenting, is something you have to do with careful thought regarding how it will affect them later on down the road. It's very misunderstood. Yes, if you never teach baby to self-soothe, you're going to eventually have problems. Yes, if baby never learns to fall asleep on his own, you're going to eventually have problems. Yes, if you never let anyone else hold him, you're going to eventually have problems.
So you make sure that your baby has balance. If he never gets out of your arms to explore, there are going to be problems. Part of attachment theory is based on the yo-yo, or you coming back to baby. When you leave and come back, they learn they can trust you. When you respond to their cry, they learn they can trust you. So snuggle and cuddle and love on him, but also teach him that he's safe when he's not in your arms. That you come back. That he can trust you.
I wouldn't pay any attention to that person you mentioned. You know what is best for your baby. But read up...there's so much about attachment theory that the mainstream does not understand. And unless you understand it and apply it properly, you ARE going to raise a little brat. It's important that if you're going to attempt to use a certain theory in parenting, that you understand it fully.
Best of luck, and congratulations on your little guy.