Pruning and lopping trees is a lot more dangerous than it is often given credit for. It can be easy to fall into the assumption that it is only working at height that poses a risk, or that working with saws and sharp tools is the most common source of injury when it is actually a combination of these factors but more so the actual act of removal that is the biggest source of accidents. There is also an inherent risk to the tree itself as cutting too much or cutting branches in the wrong way can cause irreparable damage to the tree which will result in it dying.
It is not just the cutting of trees that arborists are trained in, they also know a lot about the health and care of trees. Quite often trees will suffer damaged branches that will ultimately die and these should be removed to preserve the health of the tree and protect anything and anyone beneath it. Similarly, trees are susceptible to a variety of diseases that can cause branches, and if left untreated the entire tree, to die. If you see any evidence of broken of diseased looking parts to one of your trees then you may need a tree surgeon to check the problem and remove any dangers before they worsen.
Shape and growth
There is also a lot of understanding in ways of pruning trees that cause regrowth. You may see this around towns and cities where at a certain time of year the trees have a prune and look bare for a while before erupting into new leaves. This is a carefully executed process that skilled tree surgeons plan and carry out in order to control how the trees grow and encourage them to develop in directions and shapes that are desired by the landowners. Randomly hacking parts of the tree off is very unlikely to have quite the same effect and could actually lead to you causing more harm than good.
If you need a tree felled, then this is a slightly different matter and a professional arborist should be called in to handle the task. The biggest reason for this is safety as there are a lot of different factors to consider in felling a tree safely including the removal of branches and the direction the tree will fall when cut. It is easy to underestimate key parts of this process and cause serious injury and damage; for example branches often require removal before the felling of the tree but it is very easy to take for granted just how much a tree branch can weigh. Not supporting the branch correctly or worse misjudging where it will fall can lead to severe injury and even death. It is not worth the risk of attempting these jobs without the proper tools and training, so stay safe and call in a professional.