This is a very common problem.
Possible causes can include exhaustion, depression, low hormones (usually testosterone, thyroid or cortisol). Life today is often fast paced and packed full of things to do. Modern conveniences like dish washers, clothes washers, car washes mean that we can pack a little more in doing other things like computer work or watching TV (which in itself is bright lights and flashing objects which means the brain needs to keep track of every single dot on the screen) mean that we squeeze more in to the day than previously, and we don't like to rest or sleep! This causes the brain to get tired - it doesn't have time to recharge nor does the physical body. Depression occurs from time to time in many people. Some age groups are more susceptible (like teens where the frontal lobe is less developed and judgement is impaired, especially in boys where the testosterone levels are high).

Another common time is around "midlife crisis" time. Some people have a family history of mental illness, which might make them more susceptible as well. A lack of rest can cause the brain not to cope or recharge and therefore less able to deal with emotions. Hormone levels are also important as these act as messengers to the body. If you are worried then see your doctor and they might give you some tests to do including questionnaires and blood tests for starters and they can then work with you to help improve how you are feeling.