The behavior you described is not universal to all guys, and it's essential to avoid making sweeping generalizations. Individual behaviors can vary greatly, and not all men engage in this practice.
For those who do, there can be various reasons behind the behavior, including:
Habit: It might be a habitual action, and some people have unconscious habits that they may not even be aware of.
Curiosity: Humans have a natural curiosity about their bodies and their senses. Smell is one of our senses, and some individuals might be curious about the natural scents of their body.
Hygiene: Although it might seem counterintuitive, some individuals may smell their fingers after scratching to check for any unusual or offensive odors. It could be a way to monitor their hygiene.
Sensation: Scratching certain areas of the body can trigger sensory responses. Some people might smell their fingers out of curiosity about the scent related to that specific action.
Cultural or Social Factors: In some cultures, bodily functions and habits are discussed more openly or casually than in others, leading to different behaviors related to personal hygiene.
It's important to remember that behaviors can be individual, and not all men engage in this specific practice. If this behavior becomes obsessive, compulsive, or interferes with daily life, it could be a sign of a more significant issue and may warrant seeking guidance from a healthcare professional or counselor.