Relationships : What is Love?
There 3 things interplay here:


Kama Raga - attachment to sensual objects or objects arousing lust
Chanda Raga - attachments to people (lovers, loved one's, family, friends)
Suba Sanna - perception of beauty in the shape of the body


So when you see a person the following can happen:


Pleasure, displeasure, neutral sensation on how you perceive the person based on


Previous interaction and perception formed as friend or not or a person who matters or not or good person or bad person or likable or not
Perception of looks of the person


Relative to one's looks
As an object of desire




So when you see a person of the opposite sex the 1st time, what you get is Kama Raga and Suba Sanna. This is in seeking of pleasure born of such interactions.

Though Kama Raga heavily influences Chanda Raga, the main thing is that the person is influential in you life / perceived world. As the "puppet master" of the perceived world you get pleasure from the "puppets" in the show when they seem to go according to your expectations.

Chanda Raga is what might keep a relationship going even when Kama Raga subsides with time and into old age when Suba Sanna wanes off.

Though in seeking pleasure we get the above 3, in fact these give diverse sensations: pleasure, displeasure, neutral due to impermanent nature and non self nature of existance. All the experience you can derive from it is Dukkha (pain - Dukkha Dukkha, pleasure - Viparinama Dukkha, neutral - Sankhara Dukkha). So to understand the 4 Noble Truths contemplate on the arising and passing of sensations.