Why does anyone start building a private railway? - the only true answer must be 'for fun'. Yes, we justify it with 'for the children', or similar but fun must be the answer.
My excuse is that back in the late 1990s a random conversation revealed a chap who restored WW11 armoured vehicles but before he got hooked on that he'd seen a TMNR being offered for sale by a fairground and thought he might restore it for his baby daughter. Into the barn it went for 'later on' and the daughter was now in her 20s so a bit late for that and anyway, restoring tanks seemed more fun. Did I want to buy it? 'It' turned out to be a collection of bits, some untouched since the fairground owned it and some partially dismantled for restoration.
How could I say 'No'? - so a bundle of cash later a Land Rover and trailer delivered a load of track, some coach and loco bits and lots of semi-random parts.
That was the easy bit! The chronology here shows how things developed.