Disclaimer: I don't own HHGTTG
It wasn't love at first sight. It wasn't lust at first kiss. It wasn't even all-consuming passion at first indecent-shag-against-a-tree. It wasn't two hearts drawn together in dangerous times. It wasn't the intertwining of two souls whom Fate, Time, Space and Narrative Causality could not split. Let's face it, it wasn't a Hollywood movie.
There were a lot of things it was- determined; noisy; unstable; bewildering; crazy; awkward; and physical. To be honest, it was a relationship born out of inevitability rather than necessity. They could survive without each other, might even have been happier without each other. But at that point, in that place, at that time, it worked.
And in the end, that's all that matters.