When it comes to love, the English language bears no shortage of cliches. Somewhere between “Absence makes the heart grow fonder” and “Love is blind” is the old chestnut “Love strikes when you least expect it.” Anyone who has tried to meet a sweetheart online knows that this adage can be more fiction than fact, but there are great rewards in the unexpected love story: spilled coffee in a crowded elevator, eyes meeting on the Metro, sheepish acknowledgment as two single parents pick up their kids on the first day of school.