Sapient alien robots and humans can be friends, once they are able to communicate with each other. There is no reason this should not be, if we are to assume the existence of sapient alien robots; beings with the ability to reflect on "life, the universe, and everything" would have a lot in common.
One definite lack of commonality, however, is the ability to reproduce together. Metal robots and flesh creatures just can't have babies, no matter how hard they try. The physiology is simply not compatible. (Several authoritative works have been written on relevant aspects of human biology. Readers are advised to seek them out for further reference.) Since the urge to engage in sex is often a major contributor to romantic interest, it may seem unusual for this attraction to exist between a robot and a fleshling -- and yet, throughout Transformers fiction, such examples do exist.
To write them off as silly plot contrivances is easy. It is, however, also easy to write off large sections of the fiction as silly contrivances, which leads us down a dangerous path. Therefore we must assume that, impractical and absurd as it may be, these incompatible species must be similar enough mentally to make romantic attraction, even in the absence of sexual compatibility, possible.
A list of notable Transformer-fleshling pairs where an inter-species romance was either hinted at or outright stated, is given below.
Marvel G1 continuity
- Bumblebee and Scarlett
- Cloudburst's Pretender shell identity and the First One of Femax
- Skids and Charlene
- Skullgrin and Carissa Carr in Monstercon from Mars!
- Sludge and Joy Meadows
- Ultra Magnus and Cindy Newell
Unconfirmed G1 continuity
- Glide is known to pick up cute human girls and give them rides on his motorcycle mode.
Cartoon G1 continuity
- Powerglide and Astoria Carlton-Ritz
- Seaspray and Alana
- Lander is in the middle of a date but unceremoniously ditches his partner and rushes off to save the day after he gets a call from Metalhawk.
- Arcee and Daniel (?)
Dreamwave G1 continuity
Cartoon Armada continuity
- The 2006 Takara radio-drama/toyline Kiss Players introduced a whole line of Transformer/human pairings (that were somehow even more disturbing).
- In Revenge of the Fallen, Wheelie apparently has some type of attraction to Mikaela, though it was more likely purely sexual, no romantic.
- Some fans believe that Transformers Animated Bumblebee and Sari Sumdac may have romantic feelings toward each other later in the series.
- The fact that most of these pairings are between nominally-male robots and apparently-female fleshlings can in part be blamed on the scarcity of female Transformers. (The default assumption of heterosexuality is also part of it; the day when we have a canonical gay robot-fleshling romance in Transformers is probably still a long way off...)
- In the Prime cartoon, June Darby has expressed an interest in Optimus.