I loathed being an only child.
As a kid, we lived away from people and I had no friends, cousins, or after school activities. I played board games by myself and had imaginary friends I talked out loud to until my teens - I was that lonely.
As a teen, my parents had no comparison to other kids my age and would give me ridiculous curfews (9 pm at age 18) and ground me for something like shaving my legs at age 14. Everything I did was the end of the world, even though compared to my peers I was an angel. (No drinking, no smoking, no sneaking out, no cheating on tests... nothing like that.)
As a young adult, I had to take on way too much emotional responsibility because my father was terminally ill for about a decade before he finally died. I had no one to share that with. My mother was his caretaker and I was always the black sheep for not wanting to be at his bedside constantly praying for his soul. (I'm so not exaggerating; I had to do that many night vigils in my adolescence and finally stopped doing it after a few years.) I was also locked in my room by myself for weeks on end except to go to school, because he was ill and didn't want me around to hear him vomit because it embarrassed him. He viewed it as a shameful thing, so he didn't want anyone witnessing it. And I wasn't allowed a phone or Internet or anything because they didn't want anyone to know that he was that sick (for business reasons on his part). So basically I was left to my own devices. Having a sibling to at least share time with or the emotional stress of that, well, it would have been helpful I think. Except I don't really wish that experience on other people, either.... So I dunno.
Now, as an adult, my mother is widowed and has no family or friends except us. And I get ALL the emotional unloading, no siblings to share the burden with. I'm still in my mid-20's and she's only in her 50's so this might go on a while. She wants to chat several times a day, be privy to everything that's going on in my marriage, she even wanted to move into the house next door to us so she could be with us every day. If I ever tell her that I just don't have the resources to be all this for her, she tells me while breaking down crying that I hate her and I'm the only family she has. Which, well, is true. (Not the hating her part, obviously, the only family part.)
DH was an only child also, but he grew up with a lot of other friends and family, and he liked being the only child. But his mother too is disabled and he gets the same kinds of guilt trips as an adult now, that he's the only family she has and yada yada.