There are two reasons why I don't let my beagle say hello to everyone we pass on our walk:
1) I don't like her pulling on the lead, and it is part of her training to learn 'leave it' or to ignore passers by and their dogs unless I say its ok.
2) Whilst beagles are generally friendly, happy dogs not everyone is a dog person. Hard to imagine, but its true, and whilst most people may not mind Daisy coming up to them to say hello, its not fair to let my dog get in everyone's way or bother them - they may not like dogs and they don't have to.
If Daisy shows interest in another dog or person when we are walking, I just cheerily say, come on Daisy, let's go to keep her focus on me and not on other people/animals.
If someone wants to have a pat or cuddle with her, that's great - I make her sit first and let her sniff their palm and then she can get a pat.