Let me start by saying I am not a doctor or a therapist. I just have a lot of personal experience with different types of depression, both in myself and in my friends and family.
Depression is a serious condition that affects many people throughout the world. There are many causes of depression and different types of depression. Some causes of depression that I have found include: trauma (someone close to you passing away, break-up, etc.), drug/alcohol use, hormonal imbalances, neurotransmitter imbalances, vitamin deficiencies, and weather. Some of the different types of depression include: Seasonal Affective Disorder (SAD), Bi-Polar Disorder, Major Depressive Disorder, and Post-Pardum Depression. This is by no means a comprehensive list of causes and types, but just some examples that I could think of off the top of my head.
You may be wondering why I am writing about such a downer of a topic. Well the truth is I have been diagnosed on more than one occasion with Major Depressive Disorder, and I feel like many people don't understand these disorders and therefore do not know how to react to or handle depression in themselves or others. Some people even believe that depression is "just a case of the blues" and can be overcome with time and an attitude adjustment. The fact is though, this is not always the case. Sure, exercise can help sometimes, distractions can help sometimes, and time can help sometimes, but it is not always the case and can depend a lot on the type of depression that a person is suffering from. If a person has a hormonal imbalance (which is often the case with Post-Pardum Depression), neurotransmitter imbalance, or vitamin deficiency, these issues need to be addressed before any real change will occur. Sometimes anti-depressant medication is necessary to get a person to a point where they can manage their life and their depression on their own, but this is not always the case either. Anti-depressant medication would not necessarily be the best bet for someone, for examle, with a vitamin deficiency.
Something that we should all keep in mind is that many, if not most, forms of depression are extremely debilitating and it can interfere with a person's ability to communicate effectively, sleep, concentrate, and even eat. It can change a person's entire outlook on life. It can make a person not want to do anything to resolve their disorder, even if they are aware that they are suffering from depression.
If you are the person suffering from depression it is very important to reach out for help. I, for one, am well aware of how difficult this can be. No one wants to admit to the world that something is wrong with them mentally for fear of being labelled "crazy" or "moody" or you just don't have the desire to fix what is wrong. There is nothing wrong with reaching out for help. Find someone you care about and trust to have your best interests in mind and let them know what's going on. They may be able to help you find the right doctor and/or therapist to help you, or even just help you get motivated to help yourself. Therapy can help you recognize the signs of oncoming depression and give you the tools to help combat it before it becomes debilitating. Of course, this doesn't help 100% of the time either, which is where support from friends and family comes into play.
If you know someone who is suffering from depression it can be extremely helpful if they just have someone to talk to, vent to, or cry to. People suffering from depression can sometimes find it difficult to control their emotions, so try not to be offended if/when they unload on you. Often times they won't reach out for help, so if someone you care about is suffering from depression it can make all the difference in the world to just have someone approach them and let them know that you are there for them. They (and when I say "they" I really mean "we" because I'm included in this as well) may need someone close to them that they trust to help them identify when they are slipping into depression, and even help them to obtain the tools necessary for getting out of it such as helping them find a therapist and/or doctor, or even just getting them out of the house to do something fun. Even just having someone to tell them that they are there for them, that they care if they live or die, and that they matter is huge.
So to sum up, depression is not just "all in your head," there really is something off inside your body most times that is causing it and there is nothing wrong with asking for help. If you know someone who may be depressed, it's ok to approach them and offer help, just be aware that it could be a difficult road for them and they will likely need support from people they care about and who care about them. It may not be easy to provide the support they need, so be sure there is a network of support both for you and the person suffering from depression to ease the burden on all involved. There are lots of resources online both for people suffering from depression and for those who know someone suffering from depression. Just know that you are not alone.