Easy bruising can be caused either by weak blood vessels This can be genetic - do anyone in your family have varicose veins, or similarly bruise easily? Or systemic, brought about by nutrient deficiency. Deficiency in vitamins such as B12, C, or K, zinc, folic acid. Do you eat plenty of pastured meat, a good range of colourful fruit and vegetables - especially leafy greens - pastured butter? It's worth considering supplementation, since most fruit & vegetables, and, as a result, animals, nowadays are less nutritious due to stripping farming practices that strips soil of minerals, and due to the impact of transportation on freshness.
Thin skin could also be a factor, which can be genetic, or a result of using medication such as topical corticosteroid creams. As Michelle said, low body fat can also be a factor, but this really means an extremely low body fat level, as seen in people who are severely emaciated, and you say this has been a lifelong thing for you. Another factor is blod clotting - if blood is slow to clot, bleeding from even minor capillary damage is long enough to cause a bruise to form. Do any of your family have blood clotting issues?
Decreased tissue elasticity (and hence weakness of blood vessel walls) is another factor - do you also have dry skin, especially on the hands? Do members of your family? Do you or anyone in your family have joints issues, and get easily injured? I would recommend taking Collagen, Hyaluronic acid, and gelatin! Good, homemade bone broth is a great way to get both gelatin and plenty of nutrients and minerals. People tend to eat only muscle meat and avoid all thats left, including bones and tendons, which have all the gelatin. You could also take a blue-green algae supplement.
I would actually recommend a skin-hair-nails supplement (such as Perfectil+skin, or Perfectil platinum) - they usually have all the essential vitamins & minerals, and also extras like Collagen. I used to bruise very easily, my entire family has blood vessel issues and I also inherited very thin skin from my mother; bone broth and supplementation have helped me a lot.
Also, be careful of anything with blood-thinning properties - e.g.aspirin. Things like garlic, ginko, even fish oil are also reported to be blood-thinning but I wouldn't worry about those since you'd have to take excess amounts to be affected. Normal doses of fish oil would actually help since O3 helps tissue regeneration and prevents inflammation.
I doubt that it's any serious underlying condition, since you've had this all your life.
Lots of love :-)