There are numerous answers to this one question. The list of ideas, cultural studies (especially on children and adults that have any kind of autism diagnosis), and theories seems like a never ending rabbit hole. There is a slight flaw in all of these studies, theories, and ideas. Trying to understand why certain people are more affectionate than others is not something you can peg down, or put into statistical spreadsheets and get a direct answer. I do think it is safe to assume that a general explanation of "affection" is any physical act that elicits an emotion of love, or caring towards someone else.
Most theories seem to lean towards how we're nurtured as children as an explanation to this question. There are some families that have no issues about being affectionate with one another, and the same goes for other cultures as well. Then there are those where showing affection is just not part of the normal routine. Does this mean that those children don't feel as loved from their parents? No, it just means that physical affection was not as common when they were little. A majority of the other theories think this is due to differences in gender. Boy's get told to "toughen up", and girls tend to get more attention and comfort when feeling emotionally compromised. We would like to think this is not the case in today's society but the evidence in the studies shows that this is very common (especially in western societies).
These theories and studies state that when these children grow up, the different nurturing aspects towards the boys and the girls will impact how they show affection as adults. The males say that their female counterpart is too emotional, and the females say that their male counterpart is too distant. The studies on this topic show that in same sex relationships the majority of lesbian couples are more comfortable being affectionate towards each other than a lot of gay men in a committed relationship together. Again, we argue here that statistics are not where we find our answer. Not all lesbian couples show affection easily, the same goes for gay male couples who do. A lot of people who have been through any kind of physical or sexual trauma can find it very hard to be affectionate, whether receiving or giving it. Even the most normal thing, a hug, a handshake, a pat on the shoulder, can feel like a violation to them.
Then there's the genetic research. Is it genetically predetermined? Could it possibly be something in the genetic makeup of the brain chemistry? Whatever the genetic factor is behind the diagnosis of any kind of autism, a lot of the research shows that some of these children are just not capable of being able to understand affection. Here, again, we find our flaw. There lots of children and adults who have varying degrees of autism and who clearly understand what it means to love, or care about someone (sometimes even better than children and adults with no autism diagnosis).
If someone has a hard time showing affection, can they change? The belief is that if it’s not a medical condition, then yes, overtime. Millions of therapists, psychiatrists, and counselors believe that people can learn to get through this difficult and sensitive topic, no matter how awkward or strained it may feel in the beginning.