Knowing your gear and how to get the most out of it isn't trip ending...usually. But, if you are miserable because your gear is soaking wet and it's cold, your will to continue will likely be tested. Physical injury isn't usually the deal breaker, it's usually a lack of desire to continue brought on by whatever straw broke that camel's back.
“He is richest who is content with the least, for content is the wealth of nature.” –Socrates
Feet. That's the main complaint I've heard.
I've seen some raw feet in GA on the AT...always due to not shaking down the footwear prior to embarking on the AT.
Shin splints will halt your trek. So will plantar fasciitis. So will achilles tendinitis. I had a 5 month battle with plantar fasciitis this year and let me tell you, it sucks so bad you seriously consider having the surgery, because those cortisone shots in the heel are....errr.....kinda sorta painful, ya know?
The ones I know that bailed were on the trail from 300 to 1800 miles. Excluding not paying attention to their body and getting injured, the most common reason was it wasn't fun any more. Sometimes it happened when they saw their hiking buddies at trail days who were now far ahead of them and thought 1700 more miles was too much, for some it was having family and friends visit them and realizing they'd rather be home, and there were others who needed to hike with a trail family which meant spending more days in town until the money ran out.
Before ever setting foot on the trail, I read a book about it. In this book, the author was quick to point out the romantic ideas of hiking during the day, sitting around a campfire at night having a merry time was not going to be the way it is most of the time... that there are days when you get up in the rain, pack your tent up in the rain, walk all day in the rain, pitch your tent in the rain, etc - and that often there are no campfires because the hordes of thru's have scoured the areas around campgrounds and shelters for wood before you. That book, and the destruction of the illusion I admit I also had, did a lot for my mental preparation of hiking the AT. I can only imagine how many people start the trail and get their bubble burst the hard way, and how powerful that disillusionment can be.
I would venture to guess there are a good many who have never even thought about what it would really be like before they set foot on the trail. The movie Wild captures this perfectly when she is trying to put her pack on for the first time and when she is walking down the trail saying F--- to herself over and over...setting up the tent for the first time...trying to use her stove for the first time. A surprising number of people attempt a thru hike who have never been on a camping trip before.
I think that a lot of the drop outs started weak and inexperienced, and hoped to quickly become lean, strong, and fast. But it doesn't work that way. Instead you get injured, disheartened, and depressed because you have not properly prepared. Preparation isn't everything, but I think it is a lot of it. First, become a hiker, then go hike!
I have seen and brought many hikers to the trail at one time in my life,I would say it happens early for some because reality sets in its a lot harder than they thought it would be in all states mentally ,body,soul,Although never thru hiked,Idid hike the trail my first year well over2200 miles,and still backpack when I can,in my experience is that the thru hike is over rated vs the enjoyment of the appaliacian trail,and not just power hiking,I respect the thru hiker that completes the trail.I think the breaking point is the fact that one decides that thru hike is not the challenge or body cant do it or decide to do something else with their time
We had a different outlook,I guess. My son and I had a short timeframe due to his college graduation date and my work schedule,so we knew we could only go from Springer to Harper's Ferry. We had the normal hardships,28 hour rain,not enough prep,sore muscles,etc. But I can honestly say we never once thought of quiting,rather our worry is that we would not get done in time. We did,with a day to spare. I do think odds are much better if you hike with a like minded companion. If quiting impacts someone elses' dream,it is a little easier to keep going.