Why do very different people attract each other?
Concepts like love and attraction are not only poorly defined, but in this context are too complex to discuss meaningfully, if a scientific answer is what you are after. It is also hard to quantify 'different' when it comes to people - there are just too many variables to compare.

But, there is actually some principles and models that could be part of a more complete answer.

Total Free Energy

Both the TOTAL1 model (an extension of the famous TOTE2 model), or the free energy principle3, assert the following:


  Us humans constantly form predictions that serve as expected state, and then compare them to the actual state of the world/body (via senses).


This applies to both micro levels (say, communication between neuron layers) and micro level (consciousness).

It then goes:


  When the predicted state matches the actual state, nothing to do. When there is incongruity, we must either change our predications (models/goals) or the world (by acting).


In this view, the brain is largely a differential processor - rather than processing stimuli all the way bottom-up, it generates top-down predictions and only left with validating them against stimuli.

This is a hugely efficient optimisation strategy for the sake of metabolic economy.

It also goes to explain:


Why we stop perceiving mechanical clocks after spending some time in a room.
Why new objects (shoes, houses) excite when new but hence after peter out.
Why relationships tend to cool-off over time (and in turn why some people cheat on their partner).


In extremely general terms - that that doesn't change doesn't excite.

Now back to Friston:


  For survival sake, all living things strive to minimise surprises by forming and updating a model of their environment.


Things get interesting when survival is no longer a concern...

Diversity

Taking into account the said above, picking someone who's different from you in many aspects means your mental models are challenged more often, which could yield excitement.

I think a good example is dating someone of different nationality, say a Westerner dating an Easterner. Differences can go both way (good and bad), but it is exciting nevertheless.



1 Wang, C. and Mukhopadhyay, A., 2012. The Dynamics of Goal Revision: A Cybernetic Multiperiod Test- Operate-Test-Adjust-Loop (TOTAL) Model of Self-Regulation. Journal of Consumer Research, 38(5), pp. 815-832.

2 Miller, G.A., Galanter, E. and Pribram, K.H., 1986. Plans and the Structure of Behavior. Adams Bannister Cox.

3 Friston, K., 2010. The free-energy principle: a unified brain theory?. Nature reviews neuroscience, 11(2), p.127.