As a counselor, one of the most common questions I am asked is whether I think someone needs therapy. My response is usually that if they are asking, and willing to work, some counseling could help. Here are some indicators that you might need therapy:
If your circumstances or emotional state are impairing functioning so that you cannot work or complete school activities, or if behaviors are interfering with the functioning of others, then it is time to get help. Changes in sleep, eating, or level of functioning are often some of the first signals that it is time for therapy. If someone is too depressed to get out of bed in the morning, or too anxious to go to work or school, some therapy could help.
Regression in developmental tasks, loss of functioning, or deterioration of capacity are signals that help is needed. Kids who suddenly aren't toilet trained anymore, or older adults that stop cleaning their homes may be anxious or depressed and could benefit from therapy.
Sometimes you might just be overwhelmed, or find that it is getting harder to feel better on your own, or that you are unable to make progress when trying on your own. These are signals that it might be time for counseling. Everyone gets the blues from time to time, but when the blues won't go away and don't get better, that would be a good time for therapy.
Specific or vague
The cause for a need for counseling may be specific or vague. The death of a loved one, traumatic incidents, or major illnesses are all specific causes for which counseling could be beneficial. Other times symptoms may be more vague, but persistent enough to need professional help.
Sometimes you may just need someone to listen, or offer alternative suggestions, or have an outsider point of view. Sometimes you may just need to vent, or have someone help process big emotional responses. A counselor can help with these, and provide further help if needed.
A professional can help determine whether you need therapy or not, but you will only receive help for the amount of work you put into it. No one can force you to participate, and a therapist won't be there to help you practice new skills in everyday life. Therapy is most beneficial when you are honest with your counselor, actively participate in sessions, complete any assigned homework, and practice new skills outside of session.