I think some people have a life checklist that they want to tick off as soon as possible:
- Get a house
- Get married
- Have kids
At work I see a bit of smugness about some people that have achieved these things early. They will talk incessantly about their house/upcoming wedding, once the first kid arrives their facebook picture will be their kid, there will be pictures of their kid all over social media and so on. I think it goes more than them simply being bursting with pride and joy, there's an element of seeing this as a "life achievement" and if they have achieved it earlier than others there's some humble bragging to make everyone else see them as a success.
As well as chasing a marker of success there's also a fear of being alone amongst some people. They worry that being on their own as they get in to their 30s will bring other problems - being the "single friend" that is pitied/secretly looked down on by their peers, having pressure from their family, the practical difficulties of getting a house when single in the modern housing market. They will see others going through online dating experiences and think "well I'm glad I don't have all of that". I notice that the ones who are most vocal about promoting their status as engaged/married/homeowner/parent are the ones who are more average looking. The lucky hot people seem a lot more chilled about being single as they get older as they don't fear being left on the shelf.
I also think it's related to subconscious fear of relationships running their course. Whilst some people will meet someone who they are happy with in the long term, most will be time limited as you see as a young person, you discover areas where you are incompatible and they fester away and in the end poison the relationship so it runs its course and you move on. Break ups are hard anyway but marriage adds a cost of leaving by making it more expensive and inconvenient - this is even more so if there is a house and kids. So people who fear being left on the shelf will push for the security of locking their partner in, so they know if the relationship does run its course, at least they will be tied together and they hope their partner will just grudgingly try to make it work rather than leave them.
And this is true, they do have a higher chance of staying together. But it's not the same as being happy together. Raising the cost of leaving doesn't increase the chance of happiness it just makes it so they will stay with you and resent you. Affairs start becoming more common from people in their early 30s and you hear the same story a lot, "can't leave her because of the house/kids but that's the only reason I'm with her". People confide in others that they meet, often through work, get closer together and start to seek that excitement and rush of a new relationship whilst feeling resentful that they are trapped in a relationship that has run its course but is bound my marriage.