Well what are you doing right? Where is the tree - in or out, in what environment with regard to light, humidity, temperature? How often do you water and how do you do it? How long have you had the tree and have you recently done anything to it, e.g. pruning, repotting, etc.?

The tree is outside, with direct sunlight all day. I water it when the soil becomes slightly dry. I have the tree since December last year.

Where do you live, though?
We need to know the weather conditions, too.

Josh

Portugal

How much do you water (enough so it comes out of the drain holes?) and is the soil mostly dry or mostly wet (deep into the pot rather than just on top) before you water? Has the weather been cold lately (the tree could be mixed up and think it's November, when a lot of trees in colder places drop their leaves)? The most common reason for yellowing leaves is watering too often, but not knowing enough about your situation makes it hard to be sure - and it's important that the water go all the way through when you do it, but that the soil be allowed to dry a little (about one inch into the pot) between times.