To love your Summoner was simply a matter of course; to fall in love was an act met with pity--a story that could only serve as a warning.
Tidus never understood these whispers and thought the whole thing nonsense until truth after truth was revealed. Love a Summoner or love a dream; maybe they could somehow cancel each other and one and one could still make two instead of zero, or maybe it was only a fool's hope, but if so, he'd choose to be the fool.