I think the answer to the question of why the leaves are falling off is that your treelet may be rootbound, or that their tap root needs more room. They like tall, deep pots, and you really have to keep them from freezing for the first ten years or so, before you can expose them to light freezing temperatures. It's probably best to keep them in pots during this time, but you will have to repot periodically.
I kept one going for six years, before going on vacation and the neighbor killed it; it required repotting every six month or so for the first two years, then I got the hang of it and just got a really big pot.
My bay trees do not grow nearly so fast, and have only had to repot twice in eight years.
Also check with the nursery and get the best potting soil you can for them. A good one may even be able to 'sex' the plant for you when it gets big enough. Did you know avocados come in genders? (the usual two, thank goodness.) To get avocadoes of any description you will need two trees, or as the previous poster suggested, getting one with grafts on it of both male and female branches.
Good luck. they can be rewarding household companions. gw