Th way I see it, th word "it" has developed th connotation of "thing", "object", "less than a person." Many people (but of course not all) would find some offense in getting called "it."
"I'm not an it! I'm a human being!"
I've noticed th pronouns people use when they refer to th dogs I walk. When they say "it," they usually do so in a context such as, "Does it bite? Please keep it away from me." When they don't say "it" (usually they say "he," demonstrating our unconscious sex bias - another story) they'll say more positive things, like, "Can I pet him? What's his name? He's so cute!"
I think "he/she" grants personhood & "it" denies personhood.
Many people relate well with dogs (man's best friend, & whatnot), so they will call them "he" or "she" (but not if they hate dogs or fear them). But how often have you heard someone refer to a spider as "he" or "she"? Not very often, & usually not in th context of, "Let's smash her!" If you intend to smash a living creature, you'll probably call it "it," thus objectifying it & denying it personhood: "Let's smash it!" When someone calls a spider "he" or "she", that person grants it some degree of personhood: "No, don't smash her. Let's just let her outside."
So th problem some of us have with "it" stems from th objectification that "it" implies. Why wouldn't we call a spider "he" or "she" (or "ze" or "ey" or "ki", etc)? Why not a tree, or a rock, or a pair of boots, or a computer? Doing so shows more respect for th entities in our lives.
Think about when a person does call an object, "he" or "she," like a boat or a guitar. It shows that th object means something to th person. To some extent, th person has granted th thing some degree of personhood. I respect that. In this overwhelmingly materialistic culture we've created, we all need to figure out how to more fully love th entities of our world, living & non-living.
Thus have I spent my two cents.