Attachment is like transference and a necessary part of therapy for it to be helpful. Everyone attaches to others they have relationships with just as they experience transference with bosses, husbands/wifes, friends, etc. Attachment is why one feels grief when we or a friend move or relationships break up, people die, etc. It's basically just the natural bonding between two people who spend time "working" together (and yes, mothers and babies are "working" together just like therapist and client, husband and wife, etc.).

http://www.web-research-design.net/PDF/B&F2006.pdf