Every single person who experiences mental health issues are uniquely different, from another regardless of symptoms, diagnosis and other factors. The only expert is the person suffering, their perspective on their experience is more important than any diagnosis or label.
A loss of connection with other people is usually one of the most prominent things I see. Reasons for this are varied, however a little extra effort from family, friends and human beings in general goes a long way to strengthening these connections. Strong connections with other human beings are one of the most important factors in recovery and growth. Remember this when you are talking with someone who may be suffering, they are usually never looking for your advice, just someone who is genuinely listening without judgement.
The use of drugs and alcohol, self-harming behaviours and avoidance from social situations are so often not the cause, but an attempt to cope and self soothe. These behaviours do exacerbate the problem; however by simply asking someone to stop does not address other underlying issues. New coping mechanisms must be built and practiced for real and long term change to take affect.
Neuroscience has told us that we can re-wire the brain. We can change un-helpful coping mechanisms, but remember this takes time. It may have taken years to learn these un-helpful coping mechanisms in the first place.
Have hope, it is possible for people with mental health issues to recover and live amazing and fulfilling lives!