I am effectively an only child and hated it as a kid and still do now as an adult. I was always alone with my parents, and when my father died then it was just me and my mom. When we were on vacation I was always bored. I constantly wanted friends over or to be at a friend's house cause I hated having to play alone all the time. I asked all the time for a sibling, but that was not meant to be. I dread the day my mom dies cause I will truly feel alone at that point and I am not looking forward to it.
I currently have 2 kids and another on the way. We just told our kids about their soon-to-be sibling and my daughter was so excited I thought she'd explode. She was so happy, kept hugging me, and said this is the best news ever. She has been talking/reading books/singing to my belly cause she said she loves the baby already and it's not due till January.
As far as bickering goes, the 2 kids I have now of course have their moments of not getting along. My son loves to touch his sister's stuff when he knows she doesn't want him too and that sets off a spat. He also loves to say, "Madison did it!" when he breks something, which of course provokes a response from her.

But overall, they are so close and love having each other around. They optionally share a room and neither of them wants to move to their own room. When I do something alone with 1 kid I always get asked where the other one is and why. If one gets hurt, the other is 1st in line with a hug and kiss, they stick up for each other on the playground, etc. Seeing their dynamic (and hopefully with the next, and last, kid as well) makes me realize I did the right thing for my family in having more than 1 kid.
Of course I realize that it's not always in the cards to have more than 1 kid. #3 took a while for us to conceive and at some point we would have given up. If that had been the case with #2, we would have reached a certain point and given up as well I am sure, as it was heartbreaking to go through the TTC process so long without success.
So in no way do I feel every family needs to go to extremes to have more than 1 kid. But if you're on the fence, then I think I'd go for it.
ETA: I dont' think I was ever spoiled or selfish. In fact, compared to my husband who is #3 of 4 kids, I think I am selfless and I think part of it is that he probably got way less than he wanted/should have since his parents had 4 kids and some of that carries into adulthood. So I certainly wouldn't worry about that factor.
Also, I had a lot of friends who were only kids and they didn't seem to mind it as much as me, so I think it all depends on your kid/family.