I am going to agree with TamanduaGirl in most of this. Most primates do better in pairs or more, but I have known people who have had single primates for the entirety of the animal's life and they have never experienced any sort of aggression. You just have to spend most of your time with them.
The examples that I have are a woman who has had a single capuchin monkey for over 16 years (and she's still doing great!), and a lady who had a tamarin for around 10ish years.
Now, I am not saying that monkey ownership is for everyone, nor am I saying I think that monkeys are "pets." They are tricky and require a lot of hard work/research before deciding to add one to your family. The ones that have behavior problems are usually being housed in cages, singly with no one giving any attention to it, or people teasing it/rough housing with it. My view is, if you do the research (for several years minimum), have an adequate habitat for the critter, have the proper mindset (this is an animal not a little person or a "pet"), are responsible, and are financially capable of keeping a monkey, you should be allowed to.
It is hard for a lot of people to understand that many smaller primates and other exotic animals are just fine in captivity as long as they are cared for properly. I've known people who have kept bears, primates, zebras, and cougars, and although I would never
keep one myself, the animals were healthy and happy and never hurt anyone because they had responsible caretakers. Unfortunately, a lot of people have only had bad examples of exotic animal keeping gone wrong as a reference and as such, tend to generalize things. I can look up some stats if people want, but I think out of the thousands of primates kept by independent hobbyists, there have only been around 2% who have attacked a human with serious injury, but since the media is everywhere, you would think the rates were closer to 99% lol. And, I believe all of these monkeys were "Old World" apes such as Chimpanzee's and Baboons, which are larger and more temperamental.
Also, as a final point, a lot of primates, such as Marmosets, die within a few years in the wild (predation, malnutrition, disease, and habitat destruction are factors), whereas with careful keeping (nice/clean/spacious habitat, good diet, plenty of companionship, training) Marms can reach 10-15 years!
Ending my speech here lol
I don't want to step on anyone's toes, and this wasn't directed at anyone in particular! I just want to spread some awareness that there are good primate caretakers out there! If anyone has any questions about primates/things I've said, you can feel free to PM me, and I can direct you to more sources of info