The concept of 'thevai' (need) versus 'viruppam' (desire). The stories often start with a marriage of 'thevai'—family arrangement, economic need, etc. Romantic 'viruppam' grows within that framework.
It validates the traditional idea that love can and should grow from duty, not necessarily precede it. It's a very different model from the Western 'fall in love, then commit' sequence.