A kiss is a peculiar sort of human bonding ritual involving (sometimes mutual) oral contact. The reasons for this strange phenomenon are usually beyond the typical Transformer's understanding, resulting in many an uncomfortable line of questioning. Are these fleshlings transmitting data? Is it an energy-transferral process? Why are they doing it in my back seat?
The plot thickens. When properly administered by the correct agent, a kiss can provide an incredibly potent power boost to Transformers, with occasionally unpredictable results.
Marvel Comics continuity
It has been stated on a few occasions that the kiss is an act of atonement for past perceived misdeeds. However, this information comes from a very small dataset, specifically the individual human named Jessie. While in the company of Buster Witwicky, she has repeatedly described the act of kissing as "apologizing." Since the use of this term has not been observed in use by any other human (other than Buster himself, seemingly part of an attempt to encourage continued kissing from Jessie), the conclusions one can draw are strictly limited.
Kissing is not limited to the humans of Earth. The ritual is also practiced on Nebulos by the Nebulans Gort and Marita. The Autobot Highbrow was able to witness this event, which caused a short but violent war between their race and the Transformers. Clearly volatile emotions surround this act. Highbrow, at the time, was not able to ascertain whether kissing was akin to a binomial data transfer, and even after binary bonding with Gort, answers have not been forthcoming.
Generation One animated continuity
The Junkions have adopted the human ritual of kissing, apparently after watching much of their television transmissions. One Junkion even kissed the Dinobot Grimlock. This act greatly confused the Dinobot leader, and caused him to reassert his status as "king".
The Maximal Silverbolt and the Predacon Blackarachnia have been nearly observed kissing at several opportunities, but none of our recording equipment has been able to successfully capture it at the exact moment of contact. Again, as on Nebulos (see above), the act created an amount of irritation amongst the participants' peers. The after-effect of the kiss has been observed, however. When applied to the Maximal following Blackarachnia's Transmetal 2 upgrade, it seemed to disorient him for a moment and shoot small amounts of energy through his cranial module.
And we aren't even going to touch Rattrap kissing Rhinox after he fell into his arms..
Oh God. You don't want to know. You shouldn't know. But your curiosity will get to you. Note: We apologize for any damage caused to your psyche or immortal soul.
Live- action movie
After dancing around it for some time, Sam and Mikaela's big screen kiss moment came at the end of the film where the two were found macking on Bumblebee's hood (note: he was in car mode) to the strains of Linkin Park after the scourge of the Decepticons had ended. Also occurring while Optimus Prime gave his stirring final speech.