I've always said that "Love" has evolved due to the need for couples to stay together to ensure the offspring of their "love" survive.
I think there's a good correlation between the propensity to "love" and the ability to suffer "depression", depression helps people to stay in close groups and love/depression helps offspring survive.
All pretty logical.
This explanation is ad hoc and self-reinforcing (i.e. circular). The same argument favoring fitness could be applied to any human activity, since so far as people continue to exist any activity could be viewed as favoring "survival."
For all we know, children may be surviving in spite of their parents falling in (and out) of love; not because of it. Sexual attraction and lust clearly aid conception, but that is not what most people consider to be love. The more modern version of love, as a life-long committed relationship, is new in terms of human history, and in my view it remains an interesting (and open) question whether atempts at this kind of love benefit long term survival.
For similar reasons, I also cannot agree with your reasoning that love benefits survival specifically through the depression it might cause. Scientifically, the causes of depression are still mysterious, but there is a general sense that an increasing focus on economic specialization and job mobility, which put greater emphasis on oneself for survival through anonymous exchange (rather than personal relationships) may be increasing depression rates which is almost the direct opposite of "love" causing depression. I say this only to emphasize how trying to explain these kinds of phenomena can become circular and ad hoc.
However, even if we knew that love was the single definitive cause of depression, it still would not be a satisfactory explanation for love itself. Since evolution tends to occur over long periods, we should be cautious about trying to explain any phenomenon (such as love) with still more recent phenomena (depression).