"In languages that derive from Latin, 'compassion' means: we cannot look on coolly as others suffer; or, we sympathize with those who suffer. Another word with approximately the same meaning, 'pity', connotes a certain condescension towards the sufferer. 'To take pity on a woman' means that we are better off than she, that we stoop to her level, lower ourselves.

That is why the word 'compassion' generally inspires suspicion; it designates what is considered an inferior, second-rate sentiment that has little to do with love. To love someone out of compassion means not really to love.

In languages that form the word 'compassion' not from the root 'suffering' but from the root 'feeling,' the word is used in approximately the same way, but to contend that it designates a bad or inferior sentiment is difficult. The secret strength of its etymology floods the word with another light and gives it a broader meaning: to have compassion (co-feeling) means not only to be able to live with the other's misfortune but also to feel with him any emotion - joy, anxiety, happiness, pain. This kind of compassion therefore signifies the maximal capacity of affective imagination, the art of emotional telepathy. In the hierarchy of sentiments, then, it is supreme." -The Unbearable Lightness of Being by Milan Kundera

Tomas thinks that he loves Tereza because he feels this co-feeling towards her that he has never felt for any other woman. Maybe the better English word would be empathy instead of compassion. Co-feeling is not generally difficult for women. I guess more men find it difficult. Tomas is drawn towards Tereza because of this co-feeling but eventually it becomes a burden. I agree that pity is not love. I think some women have a tendency to confuse pity for love.