eros

Means romantic love

'in love' is to be consumed by a passionate longing for someone.Hence it is impossible for two people to stay 'in love' when they have lived together for a long time. Why? Because when want is satisfied, it ceases to be want. We cannot continue to crave that which we already possess.

Typically, when two people fall in love, they are free and unattached. They live in different houses, lead separate lives and are free to give their hearts to whoever they choose.
Romantic love flourishes under these circumstances. The time lovers spend apart makes the time they spend together all the more precious; and love is all the sweeter when it is given freely and spontaneously.But once two people commit to being 'a couple' (typically by marrying), they are no longer free to live and love as they please. Their actions and affections become bound by constraints - constraints that are entered into voluntarily, but constraints nonetheless.

Unlike eros, friendship thrives under constraint. The very stuff that chokes the life out of passion (duty, responsibility and routine) causes friendship to flourish and bloom.
'Friendship... never rises to such a height as when any strong interest or necessity binds two persons together, and gives them some common object of pursuit.'
This is why successful marriages are not characterised by swoons, sighs and palpitations of the heart - though they will have had their share of these things at the beginning. They are characterised by care, consideration and tenderness.
The happy couple, says Comte-Sponville, 'have managed to transform the passion and ardour they had in the beginning into joy, gentleness, gratitude and trust, into happiness in being together.

Friendship between husband and wife is the greatest friendship there can be: 'For they are united not only in the act of fleshly union... but also in the whole range of domestic activity.'