Hi...
Depression is a feelin of redrawn from everything. It can happen when one loses a close person in his/her life or major worries in work or social or break up. Serious depression can be lead to suicide.
Any person who is suffering from depression can feel tat the world is not treating him/her very badly, nothing is working out as it should be.
A way to see whether he/she is suffering from depression are:
1) No appetite for food
2) confine to him/herself (eg. locking in a room)
3) Not talkin to any person
4) Not in correct stage of mind
5) Always in a deep thought
6) Sudden change in his/her attitude , behaviour or daily life
7) Talkin to him/herself

Loses interest in listening to other person
Some of them have to going on for a very long period of time.
Talking is the way u can try to talk her out of depression. For talking non-stop for some period of time may try to relax his/her mind. Topics of the conversation can be anything or interesting.
For curing it, it's up to the person to overcome it. Nobody can help him/her but only him/herself to do it.
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"A relationship is like a rose,
How long it lasts, no one knows;
Love can erase an awful past,
Love can be yours, you'll see at last;
To feel that love, it makes you sigh,
To have it leave, you'd rather die;
You hope you've found that special rose,
'Cause you love and care for the one you chose."