I'll address the 2nd thing first:
No argument about there being differences between a parent/child relationship and one between two adults. Maybe the first instance of what i was getting at, that someone would think of, would be a parent/child relationship, but i was thinking in terms of "beyond that" i guess- it can be between any two individuals to me- siblings, couples, best friends, an adult child to a parent- that answer can only really be given *by* an adult, and in a non-parental context, it's not so much a "constant", as it is maybe just a single moment or two over the course of the relationship, but i was trying to get at a connection that is characterized by a sense of awe and recognition, that is not defined by role, but transcends it- just one conscious living being truly seeing another, as a fellow-traveler on the path from birth to death. As people often will remind others that these posts or chats are "just words on a screen", so too is life, in terms of any of us only being able to "know" anything by our 5 senses, and having that raw input processed through the ever-increasingly complex filters of our "thinking" minds; we are all trapped within our own "prison of the flesh", so to speak.
But this brief flash of recognition, however short and subject to fading, is that of a feeling of knowing, rather than supposing, that you are not alone, in the deeper sense of it. It is learning all at once that you are real, the other is real, life is real, the universe is real, beyond any subjective lens or filter; we can't bear up under the intensity of this and stay sane and functioning for long at all, so even the raw memory fades quickly, and the difficulties and ordinariness of life rush in again in a heartbeat. But all it takes is once, in the sense of of, say, intercepting th ET equivalent of old Howdy Doody broadcasts would forever change our collective definition of life and intelligence. That is what love is to me. I have felt this at an older friend's deathbed, during a sudden emergency at sea, with a g/f, with my grandfather, etc. When i say "i love you", i'm not looking at the window dressing: of age, height, weight, gender, relation to me, personality type, race, religion, taste in clothes, whatever. I love you for the "you" that exists independent of qualifiers; i love you because you are. This doesn't mean i won't argue w/ you, get pissed off at you, or walk around drooling like some blissed-out cult zombie- i may end up having to stay away from you entirely, depending on where life takes us- that connection may never happen again, or become buried under dysfunction, but the fact of its happening is never erased. With romantic couples, there's often a delusion that this connection should happen all the time, or that it's "supposed to be simultaneous" or some other crap like that, but things usually don't work out that way; there's more people, or couples that it never happens for at all, or only happens for one.
The love i speak of can't be "fallen into" or "fallen out of", it simply is. I can grieve over the death of someone close, or the breakup of a relationship, or be furious at a friend or relative's horrible choices, but i still love them for being, not being this or that, or doing this or that.
"...love him because he completes my world"
Notice how immediately embarrassed you got right after typing that- you got very self-conscious. But first of all, that's evidence of having had that brief flash where the "window dressing" was as nothing to you. there's qualifying words in between the "I" and the "love him", but you weren't able to interject anything between "love" and "him". "You love him"- not in the "stark" sense of because he IS, the way a toaster oven IS; of course that's ridiculous. You love him because he IS, and you love him because HE is; the HE that's ageless and sexless, not the ever-shifting daily summary of words or acts that inform your perception of how he defines himself.
I sincerely hope the following doesn't happen, but 6 months from now some change in perception or circumstance could lead to a nasty break-up, complete with mutual pain, bitterness and recrimination, and you could spend weeks walking around feeling empty inside, like you were hit by a truck, not knowing what to believe anymore. Let's say you eventually move on, regain your self-confidence, and connect with somone else. Fast-forward 20 years- so much has happened since you even last thought of him, you haven't been able to devote any energy to remember to resent the outcome of the relationship.Upon reflection, you feel badly for your younger self as well as his.One day get a phone call, he's been in an accident, and has only a short time left, and wants to make amends and say goodbye, before the inevitable. You're at a loss for words- it's been so long, the pain of your break-up comes back to memory, yet your life since then has been such, that you cannot know if it all really wasn't meant to happen that way, and you know that the brief connection you made, however fleeting, was real, and the bitterness had faded long ago.
You go to the hospital, and find he is heavily drugged up and intubated, speaking is out of the question. He stirs, sees you- your eyes meet, he squeezes your hand briefly; and the agony of his injuries, the massive amount of drugs in him, the ugliness and pain of your parting ways, the years and events that have passed since last you looked upon him and loved him without words- all these things seem to whirl away as transparent illusion- you simply love him because he is.
Enough about that, i've probably come off like a loon already, but- (i'm copying your post, i still can't believe you actually wondered seriously about this, i'm flattered to death)
I'm a laid-off blue-collar guy who barely graduated high school on time, i just do a lot of reading and thinking, and i tend to take a long time w/ a post or reply- i preview and edit a lot, to make sure a reader doesn't have to trip over too too much to get to my point. (Garsh, i'm still blushing

)
*took so long to write, i see another reply got in there*
...C'mon Katie- my head won't fit through the door now, it's so swollen...