Deforestation or the cutting down of trees in forests has many causes. The primary ones are:
The wood from trees is used in many ways. In many parts of the world it is a primary means of power. The wood can be burnt directly or converted to charcoal. Trees are also used around the world to build houses, support structures, furniture, etc. Only a small percentage of the wood we use is obtained from sustainable plantations. Most of the high quality wood is obtained from logging in forests.
The population of humans around the world is rising. With a rise in population more land is required for human settlement and also to grow food to sustain the population. Even if there is no shortage of land, forests have been encroached upon for reasons like mining, building roads, etc.
It's also important to point out the use of swidden agriculture (the use of slash-and-burn methods) to deforest land. In Guatemala, for instance, swidden agriculture is the primary driver of deforestation in the Maya Biosphere. Deforestation in this area is specifically anthropogenic (human-caused) and is motivated by illegal cattle trafficking to launder money from drugs.