It's surprisingly easy to pack the contents of your entire life into one bag when necessary.
Some jeans, some t shirts, a few jumpers, one good jacket, plenty of underwear, a white shirt, a black shirt, a pair of smart trousers, a pair of shorts, a suit jacket in case you have an interview to go to, some formal shoes, a tie, a towel, a toothbrush, some cologne, a razor,a blister pack of practically every basic medication you can imagine, a few items of jewellery some special keepsakes, condoms, lube, a belt, a phone charger, a laptop, and a favourite book you don't mind reading again and again for the lonely parts of the nights. If you have some extra space shove in a 4-way extension lead and a container of your favourite tea/coffee/whatever - these are always in short supply when you need them most. Hell if you're the sort of person who can carry a stupidly sized rucksack or maybe just only owns 3 tee shirts and 1 jumper, stuff a pillow in there too. Pillows are like gold dust for this kinda shit.
All of this fits in a 65 litre rucksack, with the exception of one of the jackets which you'll have to wear. That's okay, it's a good place to put your keys, wallet and phone that wont involve them ending up at the bottom of a pile stuff.
And that's an entire person's life, in one bag. Ready to go and movable at any point, to anywhere, via practically any method of transit. Everything else is a luxury.
Oh don't get me wrong. I'd rather have all my possessions, I'd rather have spare shirts, some other books to read, a tv and some dvds, my wok and my stockpot to cook in. And we can all pack up our life for a week or two when we're going on vacation. But sometimes you don't know where you're gonna end up, or when, or how, or what you'll be able to salvage when you get there. So it's reassuring to know that when I have to, I can fit everything about me into one bag that I can carry on my back and meets practically every travel weight and size in existence. Or it's kinda depressing if you think about it for too long. Let's not dwell on the subject...