I've never thought about giving up so I can only speak of what others tell me, and the main thread there is
lack of confidence. This is only to be expected, looking at the PPL syllabus.
Obviously there are many other factors, and we could all go and make a list.
I think the most interesting ones are those which are preventable.
Lack of money is hardly preventable, and there will always be loads of people that do a PPL but who haven't got the funding for subsequent flying.
Lack of support from one's family is likely not preventable - short of getting a divorce and getting a new girlfriend who likes flying (a very tall order IMHO!! and I am talking about the new GF, not about the divorce...).
Getting back to lack of confidence, this is preventable but is a very hard nut to crack within the present PPL syllabus and the present "airfield/school political" climate. I've done all I can for a lot of pilots by writing up detailed trip reports with tips etc, and I know from feedback these have helped hundreds of pilots. But what is really needed is
mentoring and
modernisation.
On the 2nd one, the syllabus will never change because any expansion would be unwelcome by most, especially the schools. So forget this one.
A suitable mentoring scheme would I am certain help a lot (I gather AOPA is looking at some sort of post-qualification scheme) but this is politically hard to get past the flight training industry because schools generally dislike ex students hanging around because they are seen as usurping the instructors' authority which "must" be total and unquestioned. I was pushed out pretty fast myself after I got the PPL and stopped renting and bought my plane, though I did get used for a couple of fly-outs, to ferry the pilots' wives
The actual pilots would always be in the LHS with an instructor in the RHS (in the school's planes, not mine) so that each leg of the "club flyout" was a fully chargeable training flight.
As for "messing around" with the school's existing or past customers, that is out of the question and would have to be done off the airfield. And the "post qualification" line is a tough one to stick to because let's say you meet up with 3 pilots, and one of them hasn't yet got his PPL. Do you tell him to leave the room? You would just have to hope he keeps his mouth shut. Tricky! Anyway I very much hope something changes on this because it would make a huge difference to dropout rates.
Money is not the biggest issue, in general.
The best thing for a new PPL is to buy a plane, or buy into a group. ASAP.