There are many factors and they are different for each person. But a thru-hike is very goal oriented. One must go from point A to B be in a specific amount of time, to maximize weather conditions, or other limits imposed by responsibilities at home. There are a lot of logistics involved. A fairly comprehensive pre-trip planning process including a hiking timetable, solutions for re-supply that fit into the itinerary, daily monitoring of progress versus plan, and adjustments to the plan.
Although I have never done a thru-hike, I have done several multi-month trips. To me a thru-hike is a specially focused endeavor with unique aspects. One of the biggest difference to me is the social aspect. Typically you will meet many people of a like mind, hike with new friends on and off, interact with trail angels or maybe stay at some of the communal gatherings along the trail. Most thru-hikers document their trip in some manner and often remain in contact with those they met pre-walk or during the trip. In a sense, it is part life-style with like-minded people and part adventure. When you leave the trail you leave your social support system. You leave one culture and delve into another.
Contrast this with some of my trips. When I got out of the Service in 1971, I almost flew home. That is, I landed at LAX and in the terminal realized I was not ready to go home. So I decided to go hiking. I had no gear, no plan, no pressing financial problems, and no responsibilities. So I bought some gear and went into the Sierras. I had no itinerary. During the hike I would get a Forest Service Map and hike to places that looked interesting. When I was done with an area, I would go to a Ranger Station and get a new map for a new forest. I wandered from Kernville through the Sierras, almost to Yosemite and then returned via different routes. Since I had no itinerary, I just figured that when it was time to go home I would know, the Voice would tell me. I started the trip in April and sometime in September I was ready to go home when the Voice said it was time to return. At this point, I was at least two weeks away from getting home, so I hiked to Kernville and hitched hiked to the LA area. I had nowhere to go or a place to live. I owned no personal transportation. I rented an apartment, got a part time job and spent most of my free time reading, listening to music, running or backpacking. And I was happy doing this from day 1 of my return.
In 1972 I decided to take another similar trip. The part time job was not interesting, but not something I hated either. The Voice just said it was time to go for a long walk. I returned to the Sierras and traveled to many places I had not visited the previous year. Same plan, I would know when it was time to go home, when the Voice told me; not some arbitrary time frame, or when I reached a point “B” but when I was mentally ready. Again, I finished in mid-September when the Voice said it was time to go home. The difference this time was that I found a job I loved, and was happy working and did not feel the need for a long trip. I kept busy with many shorter trips and was perfectly happy with everything in my life.
In 1979 I decided to take a 3 month (or longer) cross country motorcycle trip with my first wife. We had sold some property, had money in the bank, no jobs, and no urgency to get jobs. We bought a small tent trailer we could tow with the motorcycle and took off for the east coast. Now this trip had a vague goal, and logistics were easy, because the bike enabled us to get supplies without difficulty or planning. Our only real plan was to stop and visit relatives in Ohio and New York. Now spending an extended trip with another person took a little adjustment for me, because I could not do only what I wanted, but had to come to a “couple agreement” and consensus at times. Also we did not have the social interaction with other bikers like one would have on a thru-hike. We might meet someone in Truth or Consequences, NM and then see them a couple weeks later several states away. These were just passing moments and no relationship was established. Also important was the flexibility. There was no set timeline or route. Once we got to East Rochester, NY I was no longer interested in actually touching the east coast, the Voice told me to go see Niagara Falls and the Soo Locks, so we turned around. The loose plan to head west towards the Canadian Rockies after visiting the Soo Locks. We took a ferry from Tobermory to Manitoulin Island and spent time in the area including inspection of the Locks. Plans changed, as the Voice told me I should be riding in the States, not Canada. So we headed south back to the US and spend a lot of time in Wisconsin and Minnesota traveling the more remote byways. Crossing the continent west, I still did not hear the Voice saying it was time to go home. When we got to Lake Tahoe I thought we would go south to Hwy 395 and then home. We spent a few days around Lake Tahoe, but no Voice. So we drove west to the California coast. Decided to visit my brother in San Luis Obispo for a few days and then headed home. But the Voice was not there telling me it was time to go home. I was not ready. Once we got home, I realized it was not time to be there, so we took off on the bike again for the Sierras. Arriving in the Mammoth Lakes area just after Labor Day we were snowbound for a couple days at Lake Mary, too dangerous to ride a bike out and too cold for our liking. Once the weather cleared and the roads were plowed, we headed south and ended up along the Kern River. We spent a week or two (can’t remember) along the Kern in Indian Summer type of weather. Then… the Voice told me it was time to go home. We packed up, drove home and I got a job. It was time. And “re-entry” was a non-event. Perhaps a thru-hike is so planned that the hiker does not have a Voice to guide them?