A Lilac a Day, Day Eight

I've often wondered why some trees, like this one, seem to be covered with blossoms, while other have virtually none. I initially assumed it was due to how much sun each tree received, but it can also be due to improper pruning. Lilacs bloom on the growth from the previous season, so they should be pruned within a week or two after the flowers have faded. They can  also be over fertilized (they don't like a lot of nitrogen). And, of course, they do need a minimum of  six hours of sun light each day.