Sir..whenever im brushing my teeth blood comes out..why so?

Gum problems?: You most likely have gingivitis or periodontitis, both different stages of gum disease. There are other possible causes. You need to see a dentist for a one on one clinical exam, x-rays, diagnosis and treatment. Even in the absence of pain, left untreated, any of these issues will only get worse over time.

Most Likely Perio: Without more information i can't be specific, but the most likely answer would be a form of periodontal disease. There is also a stress induced disorder called anug (also known as "trenchmouth " from ww1 soldiers would develop while in the trench's of ground warfare) which can cause gum pain throughout mouth. Best idea is to see a dentist asap for an exam and diagnosis.

Meds and diet: Any medications, diet, home care, other health issues can also contribute to this.