I'm not talking about an angry boss, or worrying about how to pay for college tuition. I'm talking about things that might harm your tree or make it unhealthy.
Give your tree the best fighting chance!
When a tree is obviously damaged by wind, lightning, a car-crash, fire, or root destruction by construction equipment, It's clear that action needs to be taken to either nurse the tree back to health, or to remove it if it is irreparable and dangerous.
There are other issues that might stress your tree and give no clear indication that there's a problem. For example, the soil may become compacted, preventing proper water absorption, the water drainage might be changed, your tree might be affected by airborne pollutants or salt water. Some signs of this subtle stress are yellowing leaves, smaller than normal leaves, dropping branches or limbs, developing a staghead or deer antler appearance.
Once a tree has suffered some kind of stress, whether quick and severe or more chronically, it can fall into a stress cycle, much like a sick person who becomes less capable of fighting off infection. Bacteria, virus, insects and fungi that wouldn't normally pose much of a danger to a healthy tree may find a foothold on a stressed tree, thus introducing even more stress, until ultimately the tree dies.
Just like human medicine, the best treatment is prevention, but when that fails, trust a certified arborist to asses your trees, save them if possible, or safely remove them if necessary.