I feel always tired i don't know why ?

Tiredness-sleepiness: Tiredness or fatigue has many causes, including anemia, thyroid disorders, chronic infections, heart disease, depression. However, people often complain of being tired when they have difficulty staying awake or daytime sleepiness. This can be related to not getting enough time in bed, sleep apnea, poor sleep at night, narcolepy, idiopathic hypersomnia, etc. See your doctor about this.

See below: Agree with Dr. S. You need to exclude depression. Despite it sounding counter-intuitive, regular aerobic exercise may help. Have a healthy diet, exercise, drink plenty of water, have safe sex, and get HPV vaccine. Wish you good health!