Pets are like babies that way, you get to pet them and hold them. With older children and adults there is much less contact, unless you crash into them. Most people you cannot hug and caress the way you can a pet. With a pet you can freely share affection and contact without any of the awkwardness or potential rejection involved in most human relationships. One exception can be a significant other, but those are harder to find than pets and can more often involve conflict and uncertainty where the pet is generally 'old faithful'.

That ties into the reciprocation. The pet will generally not say "get lost, I am playing a video game/reading/watching TV/washing my hair or doing any of 101 activities that are more important than you" or "get away from me you geek" or "geez dad you are embarrassing me, what are people gonna think?". Even if you are ugly and your mother dresses you funny and your jokes are stale and poorly delivered, you generally get a good reception from a pet. They will purr or wag their tail and jump up and down (okay, cats won't (what do people see in them anyway?)) and play fetch or pretend fight or chase. Generally they give the owner/friend the message that "you are awesome. I totally love you."

It's nice to hear that every now and then, or at least fool yourself into thinking you hear it when the dog is really saying "I hope you got me some of those biscuits that I love so much" or "I know you are not gonna eat all of that pizza by yourself" or "get the keys and take me for a ride (or walk (or both))"

Third, I suppose, are opportunities to play and be entertained by antics. Whether just playing around with a string or chasing a ball or wrestling/tearing around with another animal or doing the suppertime or car-ride dance, pets provide hours of non-competetive play and entertainment. Their joy of living rubs off on their owners.