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Maybe the entire idea of love is completely fake in that you don't feel a connection with another person because of some innate, natural desire to be "in love" just because it's "love". 
And this goes with compassion and kindness too. Maybe when we let that old lady sit in our seat on the bus, it's not because we "feel compassion" for that person even though we think we do
It's because it's what is socially accepted since if you don't then you are afraid of being judged and criticized and if you are religious it's maybe because compassion is taught in your doctrine and youre afraid to be punished by god or other reasons. 
Those are extremely selfish motives. 
Also, if youre alone with someone and you can choose to do something kind to said person, you would do it either to please said person or because it's so engrained in you that it makes you feel morally superior and boosts your ego but we would pretend that it is because of moral, collectivist, true compassionate reasons.
Everything we do has a somewhat "selfish" root to them, no matter how much we deny it. Every altruist action comes from a place of wanting to fit in or gaining trust. It also works from a psychological viewpoint too since making someone happy means they're more likely to do the same to you in the future, and doing what you feel is "right" gives you a sense of self acceptance, which also results in a happy feeling. Grieving works as a mechanism to cope with the loss of someone in our "tribe". Since humans are social animals who work together to survive, a loss of a member means more chance of death
Also idk nostalgia seems to be a much more complex feeling or idea about events and emotions not just a person.