Eros is not tranquil - it gives us spikes of happiness rather than a constant feeling of wellbeing. It's the love we feel at the beginning of a love affair and corresponds to the expression 'falling in love' since it is as involuntary an impulse as a physical fall.
Very different from eros is philia, a serene love much more akin to friendship, with its reciprocal kindnesses. You love each other for the happy experiences and pleasures you share.
