most of the time my friends (really friends, not just casual acquaintances) are the ones who reject my offer to lend them my lenses.
that's how i know they're really my friends, coz they won't risk the friendship over the chance that they will spoil my (expensive) gear. some who do borrow or take up my offer to lend, are themselves photographers who know the value (monetary or otherwise) of my gear and get over-protective of my stuff when it's already in their care.
of course if it's just an acquaintance or a friend known for his careless ways, i'd just say i'll be shooting that day/week/month and will need that gear. which is also probably true so not exactly lying. even if i didn't plan on shooting that day, i'd end up shooting as an excuse.