Those cold/wet days where it's a real battle to get out of the house, drive to the trails and get out on the bike, but 15 minutes later you're wondering why you ever doubted.
Loamy soil and drifty turns
A herd of deer that only notice you when you're within 20 yards, before collectively springing, running and bounding off through the ferns into the trees
the one ride in every ten, when your lungs are clear, legs are strong, eyes are focussed on the trail ahead, bike's working perfectly and you're truly 'on it' - feeling the terrain through the tyres
When you just have to stop to take in the beauty and silence around you.
Mid-summer in the Wyre forest. Low winter sun on Cannock Chase. Halfway down an alpine descent. Sat on the side of the midway singletrack climb/traverse of the Megavalanche being overtaken by a whole string of riders because everything had gone lightheaded and blurry, before waiting for a decent gap and nailing the downhill, trying to catch and overtake all those who had just gone by.
I think half of those reasons are why I enjoy riding alone as much (if not more) as riding with mates.