Dogs tend to lick human wounds and bruises for various reasons, here are some of them:

Natural instincts:
Treatment and care: In the wild, dogs eat and lick their wounds or those of their family members to clean and disinfect the wounds. It is part of their instincts to nurture themselves and their group.
attention:
Attracting attention: When a dog licks an owner's wound, it may be trying to get attention. The licking can be a way of saying "Hey, I'm here, look at me!"
Expression of love and closeness:
Bond strengthening: Licking is also a way for dogs to show love and closeness. They see humans as part of their group and want to nurture the bond.
smell:
Olfactory exploration: Dogs use their strong sense of smell to explore the environment. When there is a wound, it can be something interesting for them, so they tend to lick it.

sense:
Condition exploration: Licking can help the dog feel the condition of the wound and check if there is pain or if help is needed.
Stress relief:
Relaxation: Licking can also be a way to calm themselves when the dog is in a state of stress or anxiety.

Self care:
Cleaning: Dogs use licking to clean their wounds. They try to remove dirt or bacteria from them that may enter the wound.
Sensing: Licking can help the dog sense the condition of the wound and see if it needs further treatment.
Pain relief:
Relief: Licking can provide a feeling of relief and pain reduction. Dog saliva contains enzymes that can help heal certain wounds.
Instinctive behavior:
Natural instincts: This behavior is part of the natural instincts of dogs. In the wild, dogs would lick themselves to maintain their health and clean wounds.
attention:
Seeking attention: Sometimes dogs lick wounds of humans or their own to get attention. They may want to be treated or treated.

Abandonment Anxiety: Some dogs tend to lick wounds when they experience anxiety or stress, and this can become a compulsive behavior.
Summary
Licking wounds and bruises is a natural behavior of dogs, but it is important to keep an eye on it. If the licking becomes compulsive or if the wound does not heal, you should consult a veterinarian.