What makes a sight or an image mesmerizing and irresistible? There are some examples of this.

What makes us:


Feel we just HAVE TO watch that video or an image again, it's common with comedy videos
Feel we need to stare at a such a beautiful sight, a person or an image


It's not that something is particularly interesting to the conscious mind, but yet we still can't resist but watch.

What's the science behind this process?
Setting aside the encyclopedic issue of what might make an unspecified stimulus funny, beautiful, or otherwise pleasing for its aesthetics, it's at least evident that you're asking about reinforcing stimuli that provide incentives via emotional reward(s). It's not clear how you expect to rule out conscious interest, but reinforcement learning may not necessitate consideration of conscious control, so I suppose we can disregard the whole issue of consciousness at this level of generality (or ambiguity/vagueness). That is essentially the approach of radical behaviorism, after all.

Aside from the tags and links included so far, there's plenty of science available on psychological reward:


Relevant neural structures include the ventral tegmental area and nucleus accumbens.
Dopamine is probably the most relevant neurotransmitter, but other catecholamines such as adrenalin and norepinephrine may affect impulse regulation and attention.
Relevant personality factors include the behavioral activation system and sensation-seeking. Other factors become more or less relevant depending on the nature of the stimulus, e.g.:


Openness to experience affects aesthetic preferences, especially in art and culture.
Cultural background affects interests, preferences, and incentives through taste acquisition.
Emotional stability and extraversion may affect attitudes toward arousing or social stimuli.
Sexual desire may be relevant, depending on the stimulus and the individual.
Emotional and physiological states affect the strength and relevance of other needs / motives.
Conscientiousness or control affects impulse regulation, which is relevant if urge resistance is.

We have a dedicated question about humor: How does our brain consider something to be funny?