Well when you're a teenager I suppose it's more about sex and 'love' and the thrill of it - mixed with just 'having' a BF/GF and being 'normal'.
But as you get into your 20's and beyond it changes. I'm mid-to-late 20's and people around me are now starting to settle. There are people getting engaged/married/buying homes/having babies. They've graduated, they've got decent careers, and now they're looking to the future. I think coming towards 30, people are looking for security - in every way, emotional/career/financial/personal etc. In your 20's post uni you're working full time and believe me - it's hard to come home on your own after an 8 hour shift to a quiet empty house/flat. No one to talk to about your day, no one waiting for you, no one who missed you or would miss you if you didn't walk through that door, no one for a hug, no one to have dinner with around the table, no one to laugh with about something funny you saw that day or cry with about something terrible you saw and no one to lie next to in bed. It's lonely. And in that situation when you're working, earning and you've finally got that good career and house/flat...when you have all of that and you're single/on your own...you have time alone to sit and think "what does any of this even mean without someone to share it with?".
So, yeah. I used to think exactly the same and thought I would be happy to be single forever. But that changes as you get older and see people getting married, having a family etc and you've just got few fish and the biggest commitment you have is your phone contract.
Honestly, it's just nicer to have this journey with a side-kick. Life is pretty tough and it's hard to travel through it all by yourself. You'll really get that once you have your independence and all the material things you've dreamed of.