* Depression plays a large role in suicide. Suicide might seem like a way to end the pain, since the person may feel hopeless and have trouble imagining another way to relieve the extreme suffering that he or she is feeling. This difficulty imagining a solution to the pain partially comes from the distorted thinking caused by depression.
* A medical condition that is debilitating, painful, or life-threatening may be a reason some people want to speed the process or choose their own terms.
Having suffered a loss or major adverse life event, like separation, divorce, or the death of a loved one, is another risk factor.
* Sometimes the pain of abuse, whether it is physical, emotional or sexual, drives people to consider suicide.
* Abusing drugs or alcohol can also make people more vulnerable to depression and the feeling that life is not worth living.
* People who have a mental illness like schizophrenia or bipolar disorder are also at a higher risk for suicide.
* Gays and lesbians, especially teens, are at higher risk of suicide, usually because they feel misunderstood or rejected by their families.
* Antidepressant medications can worsen depression or cause the emergence of suicidal thoughts.
What are some of the signs that someone might be suicidal?
It can be a frightening experience when someone you care about may be suicidal. You might just have a gut instinct that something is wrong. Or the person might have shared that s/he has been thinking about suicide, or even has a plan. Some risk factors are:
Have the person's circumstances changed recently? Was there a breakup of an important relationship, such as a separation or divorce? Has the person survived—or has someone close been affected by—a traumatic event, such as a natural disaster, physical assault, or a crime? Has the person recently become unemployed?
Has the person been struggling with a difficult situation, such as a chronic illness, physical, sexual or emotional abuse, long-term unemployment, or loneliness and isolation? Does the person abuse alcohol or drugs?
Has the person had problems with depression? Are these signs apparent now: loss of pleasure, a sad mood, a change in sleeping or eating patterns, feelings of hopelessness or excessive guilt? People struggling with depression are at a higher risk for suicide.
Has the person been diagnosed with an illness like bipolar disorder or schizophrenia? People with these illnesses have a higher risk of suicide, and circumstances can change rapidly.
National Hopeline Network: 1-800-SUICIDE
Have the person's actions changed recently? Has he or she talked about suicide or expressed suicidal feelings? Made a plan for suicide? Does the person seem to be putting things in order, giving away possessions, making a will? Has the person's behavior been changing recently? Is school or work performance declining? Is he/she withdrawing from friends or family, spending more time alone?
What should you do if you think someone may be suicidal?
Trust your instincts. The person may or may not show some of the signs above. Even if you're not sure, don't be afraid about asking the person if he or she is feeling suicidal. You won't be putting ideas into someone's head, but instead are bringing out a topic that has most likely been thought of already.
Notice the level of action.
Does the person have a plan for suicide? Having a plan means the person is that much more serious about intending suicide.
Does he or she have means to carry out the plan? Talking about pills or guns is much more serious if the person actually has them in possession, or has access to them.
Does the person have a history of prior suicide attempts? The more times a person has already attempted suicide, the greater the risk of succeeding.
Get involved. Some people worry that if they "butt in," the person might be angry with them. While this might occur, you would undoubtedly rather have someone alive and angry at you than wonder later if your involvement might have prevented a death. If someone is feeling suicidal, the most important thing you can provide is support and unconditional acceptance.
* Recognize that he/she is in almost unbearable pain. Listen to the person, and accept feelings without judging or discounting them.
* Tell the person you care about him/her, and want to help make a plan to feel better.
* Call a crisis line yourself to get information and suggestions – counselors can give you some tips as well as some starting resources for developing a plan to get the person the care needed. This might be developing a support system, counseling, or getting in touch with the treatment team if s/he has a mental illness.
* If you can't develop a plan and a suicide attempt seems to be imminent, don't wait – get help. You could be saving a life. Call a local crisis center or dial 911 if it is an emergency. Do not leave the person alone.
Be sure to take care of yourself as well. It can be very scary when someone close to you is contemplating suicide, and can be difficult to talk about. Find someone that you trust, whether a friend, other family member, religious member, or counselor, to share your feelings.
source:
http://www.helpguide.org/mental/suicide_prevention.htm 
What are some of the reasons that people consider suicide? Who is at risk? (article) 
