When you buy a pint of your favorite ice cream for when you want it, it will be there when you actually do want it. It can sit there as long as you don't want it until the moment you want it. You won't decide to have your ice cream, get all ready to have some of your favorite ice cream, only to discover that it's not there because SOMEBODY decided they wanted something sweet and ate it at 2am while they were having insomnia.
... Not that such a thing has ever happened to me or anything.
Well, let's here... I have a husband and four kids. My kids are expensive and my husband is expensive. Even though we have the ability to have a dual income household, he's more of a clothes horse than I am so he costs more money to maintain than I do. If you are unmarried and childfree, you will just have to worry about feeding and clothing yourself. You'll never say the phrases, "Why is every light in the house on?" and "I'm not paying the heat the neighborhood!" There would not be tuition to consider and emergency savings would not have to be large as a requirement, but rather a preference. Life insurance wouldn't be as pressing of a thing to have. You won't have as many birthdays to buy for.
There's also less mess. If you're single and childfree, you will never have to change anyone's diaper. You will lack of a partner who may age and develop incontinence issues. You won't have an infant to care for if you're childfree. So your life will be fecal-free with exception of whatever you wish to do with your own. Any sickness that happens will be your own. Not only in matters of incontinence but with any condition (eg. dementia, diabetes, arthritis, etc) you will not be responsible for another person.
You will only be accountable to yourself. Want to waste $5500 on the best television in the whole world? Do it. It's your money. Who's going to tell you "No"? Who is it really going to hurt if you max out your credit card and take five years to pay off the balance? When you decide to go to London for the weekend, there'll be nothing to stop you. Spend $2000 on a weekend trip to Europe. No one can tell you that you can't go. If you don't mind not having anyone to share it with, the world is your oyster!
Your path is the only path. Along with the previous point is this related one. You won't ever have to accommodate someone else's dream and decision. Everything that you decide to do will be yours. At no time will you have to delay going to law school because the love of your life has decided to attend the most prestigious circus college in Belgium.
Less intrusive, rude, annoying people that you have to pretend to enjoy and love because they are Family. Almost always with marriage comes in-laws, which is another group of annoying people who have opinions about how you live your life, too many inappropriate questions, and an infinite number of jokes that aren't funny. Have children and those in-laws become even more glued into your life because they always have to see the baby and know everything about the baby, and share their opinions about how you're parenting, inappropriate questions about your life as a parent, and jokes and stories about your partner as a baby that you really didn't want to know.
It's only your stuff. When you are married, you have your stuff with someone else's stuff. After a while, there's new stuff and that's considered "our" stuff, but with new stuff, old stuff has to leave and some of that old stuff will be your stuff. I live with a shot glass collection and ceramic statues of birds. I wish you could feel how painful that is for me. I don't like tacky things. It could be worse. With my ex-husband, I lived with an exhaustive pink anime collection and japanese swords. When I was single, it was just my stuff. My tasteful, sophisticated, clean-lined stuff. Mind you, when you have kids, that's more stuff. And their stuff ends up all over the place, especially where it does not belong.
Most of the time, the stimulation within your living space will be caused by only you. You decide when the television is on and what volume. You choose the music. There is no yelling, no loud noises or bangs. If you want things quiet, you can have them quiet. When you are married and/or have children, there are individual people who are separate from you in the same living space. They will make noise and want to engage in activities that make noise, regardless of how you feel about it.
That's off the top of my head.
Mind you, I wouldn't trade my kids and husband for the world. I love quiet but I hate when they're not here disturbing that quiet. I'm too used to them being a part of my life that I can't function well when they're not here. But you didn't ask people to list all the positive qualities of being married and having children. You wanted the opposite. So there you go.