What is socialization?  Socialization is NOT  meeting dogs and people  and being friendly, as people commonly think . Socialization is teaching your  dog  to be calm  in the presence of  triggers, what ever they may be. Puppy classes, while valuable, often miss this vital skill .   

Therefore  failure to teach  disengagement  from  triggers  will have consequences  in adult life  when a  dog has developed the expectation of playing  with  every dog it sees and  becomes frustrated and reactive  when it  isn’t allowed   to  do so , or remains  shy or fearful of  others.

All potential triggers  that the puppy will be  exposed to over it s life should be introduced to the pup in the first 4 months of it’s life  if possible  , before the pup  goes through it’s default  fear period .  This can be difficult  while pups  are being vaccinated , but exposure to other  dogs,  while desirable if those  dogs are well behaved sociable and calm ( but not if not ), is not necessary to socialize. Simply  teaching that triggers eg a dog barking in the distance  or seen on a walk ,  are the cues for  positive  things  to happen  and pup should disengage  from them  and focus on owner or be calm,  will build positive  associations and  desirable behavior.  

A bad experience is worse than no experience .

Puppy consults, while  not offering  puppies to play with,  teach  you how  to teach your  pup to disengage  from  all triggers,  which  is  an invaluable skill  for future life .