From a zoological perspective, when the male and female form a "pairbond". Essentially, we just use the word 'love' as a subjective and glossy synonym. Both mates feel an 'emptiness' when separated from each other. In essence, they have 'hijacked each other's brain'. For eg- at intervals of 3-5 minutes (the average time for copulation) of the female not being around, the male will simply begin to ask: "hmm, where is she? I better go have a check on her" (Unless he is at work etc, and then he thinks about getting home to her). Females have similar processes. The 'pair-bond' essentially holds because of the allignment of both specimens survival and replication value for each other.
And people say loves complicated...pfft.