Me and DH feel exactly like this!!
I am 54, he is 51. We both just had a month off work, and have come back down to earth with an enormous bump. Being off and travelling has made us realise even more, what life could be like if we weren't tied to work.
I think it's a combination of things as you get older :
Your parents die, which makes you think of your own mortality (why am I doing something every day that I hate, when I could be on a beach in Jamaica?) My Mum died when I was 50, and it has changed my outlook.
You recognise a lot of bullshit politics in the workplace, that are meaningless.
The youngsters can be full of shit/office jargon. Who wants to listen to a 24 year old, who is permanently attached to a water bottle, spout on about anything?
You are close to being able to leave, so you're thinking about it more.
You have more money, so could afford to do things, if only you weren't tied to work, whether that be a nice holiday, or just drinking at the pub on a Sunday, knowing you don't have to up for work at 6am on Monday.
Children have grown up and left home - often they live far away and if you were retired you could visit more.
Friends around you start to retire and do fun stuff, which makes you envious - I have 2 female friends who have retired in the past 12 months. One is 54, one is 56. The change in their personalities has been shocking. Right back to how I remember them in their 20's. They are SO happy and carefree.
My DH works is a very stressful job in Emergency Services, and the shifts are terrible. He often says to me as he leaves "ring me if we win the lottery, cause I'm walking out".