1. You resist the temptation to send each other passive aggressive messages
When you get home, ready to make a complex spaghetti bolognese, featuring pancetta, mince, carrots and celery (yes, celery. Nigel Slater knows what he's talking about) and you discover that the pancetta has gone from the fridge, you resist the urge to send a message to your other half to ask "Have you eaten the last of the pancetta?!" because a) If you start sending messages like that, where will it end? Will you start sending emails saying 'What do we do when we finish a toilet roll? WE REPLACE IT' or leaving hand written limericks on the fridge that read: 'There once was a man from THIS FLAT, who ate the last Muller Crunch Corner and I'm really quite ANGRY ABOUT THAT' (I didn't say it would be a good limerick)? You really don't want to be that guy. And b) because if you didn't eat it then clearly the answer to that question is yes.
2. You struggle to throw away dating mementos
You're rooting through your bag of an evening commute, searching for a tissue like the 29-going-on-79-year-old that you are, and you stumble across the cinema ticket from the night the pair of you went to see Still Alice (and now you realise why you're struggling to find a tissue - you used up every last one whilst watching that). And in the interests of a tidy handbag and minimal cries of "OH FOR GOODNESS SAKE, WHY CAN I NEVER FIND MY KEYS IN HERE?! THIS BAG IS LIKE A RABBIT WARREN FILLED WITH OLD PAPER!" then it should go in the bin. But you pause because that was a particularly nice evening. You wore something leopard print, as you do when you're making a real effort, he was forced to admit upon tasting your main course that it was indeed spicy enough to be deemed a real curry, and not just a 'bowl of meat and creamy sauce' like he'd assumed, and the film was outstanding. Nights like that don't come around every day of the week and you find it hard to put any evidence of it in the bin, even if it does make your handbag unmanageable.
3. Sometimes, for a treat, you sit on the same sofa
When you first start dating, not only do you insist on always sitting on the same chair, you will sit as much of yourself on top of the other person as you can. But then time starts to pass and with that comes the realisation that, actually, the most comfortable way to sit is alone, and with as much room to stretch out as possible. But sometimes, perhaps if it's somebody's birthday or a bank holiday weekend, or you've got a particularly good box of chocolates that you definitely want get your whole half of; you'll sit together on the same settee. And it'll feel like a lovely, if slightly cramped, treat.
4. If they send you a text message, you don't delete it
...not that you ever did, of course, it's just that these days, with all the other ways in which friends and family can get in touch, text messages are generally only ever received from PPI compensation people, or companies who are absolutely determined to give you £2,000 for a car accident you haven't had (you'd remember something like that, wouldn't you?) So if you do get one from your other half, it's extra exciting. Perhaps they'll ask what you'd like for your tea, or send words of encouragement to help you through a particularly trying day, or maybe they're writing to compliment you on your excellent choice of attire, regretting that they were so caught up with trying to locate their wallet/tie/other sock this morning to say so. Either way, you're very pleased to hear from them.
5. Every now and then you wear proper clothes at home
The greatest thing about a night at home is the opportunity to spend it wearing the cosiest items of clothing you can get your hands on. It may not be the kind of attire you'd wear to a pub (unless your toilet broke in the middle of the night and you were left with no other choice than to go there) but you don't care because you're just so comfortable. You've both got your own set, and wow do you each feel smug when you're wearing them. But sometimes, when an occasion calls for it, you'll both pop on an outfit you'd be happy to be seen in outside, if only to remind each other that you do indeed own clothes that don't include an elasticated waist.