Imagine you were going out with someone and didn’t buy them a present just because you hadn’t reached the four-month milestone yet; imagine the uncomfortable birthday date – there’d be one, because they’ve introduced you to all their mates – and you have to confess (or more likely, studiously avoid the fact) that you hadn’t bought them anything.
To be frank, if you don’t get them anything for their birthday, you probably won’t reach the next milestone.
To translate for the current economic climate: moving into a slightly nicer flat without any intrusive housemates around to walk in on your “romantic” cooking sessions. If you first met this boy or girl at 21 then no way.
If you’re in your mid-twenties then it’s a scary possibility that two could become one sooner than you think.
You’d have a shit time and have that first flicker of doubt (“this going out stuff was more fun when I was single”).