Short answer: Because I'm mentally wired that way.
I view 'beloved' as a subset of 'friend', which is how it should be, I think. I don't especially believe in dating strangers if I don't think they're going to turn out as friends. In order for me to fall in love with someone, that person must be either a best friend, or someone who I think capable of becoming a best friend.
Note that my sex drive is not fully synchronized with my friendship database. I can lust after people I don't know. I don't have to lust after people that I know exceptionally well and like a lot.
I tend to have brief, lustful crushes on new friends who I think might be even slightly compatible. This doesn't fill the category of 'beloved', though I do expend a lot of energy on the crushes before the interest burns out, and everything settles into comfortable friendship. Or not, depending on the person.
A lot of love boils down to caring and loyalty. I care about my family, and we stick by each other. Even though we aren't particularly close now for reasons of distance and time, whenever we get back in touch, we renew the friendship. Some friendships last for a while, then fade out, and taking the time and trouble to renew them isn't worth it. Some friends, if we lose touch, the friendship springs right back up as if we'd never been separated after we get together again. Ginger and I are that way. Sue-bug from back home and I were that way the first time we met, gossiping and catching up in a way that makes me suspect that we'd already known each other before we were born, and were just catching up on the gossip about the stuff that had happened since we'd seen each other last.
I can have any number of friends. Not all of them have to quit being friends once they go out of scope. Some of them will remain friends of mine for as long as either they or I exist. I can have more friends than I have attention for at one time.
I can have more than one best friend. For me, 'best' friend is defined on degree-of-closeness. There can be more than one person equally close to me; the degree of measurement it would take to determine which person is truly closest is more than I care to undertake.
In order to stay a beloved of mine, a person must become or remain a best friend. Lust is involved, loyalty, trust, caring... the same characteristics that hold a good solid family together, plus the lust. The more intensely someone is a friend, the deeper I fall in love with them if I judge that we'd do well together, long-term, and very close. I base my love on enduring characteristics: trustworthiness/character, sense of humor, sense of wonder.
If I'm wrong about somebody, and they're not the kind of person that I thought I was falling in love with, my love fades out. When someone changes so that the deep, important parts of them that I love, are no longer there, my love fades out. If someone cares nothing for me in return, my love fades out.
I don't unlove my sister if we get a new baby. I don't have so many spots in my heart for 'sibling', and anyone more has to just deal. It doesn't work that way. One may wind up to be my 'favoritest', but everybody's still there. I only have so much time and attention to go around, but that happens with everything.
I don't hold with the idea that one must love one's beloved one way, and love one's family another way. Certainly, one would be generally foolish to lust after family, but if you wind up marrying your beloved, they will be family, and the most important part of it. They're just family that you get to choose, rather than the family that you happened to be born to. If you have a good family, the sort who don't cause angst at holiday gatherings, the sort you love to be with, it's very little problem knowing when you have found someone who ought to be family too. If you weren't born with a family like that: first, I'm very sorry; second, you may find that certain close friends are the family you wish you'd had. That sort of close friend is the sort of person who makes a good beloved.
Now, back to lust. I don't have particular trouble with having sex. I like the person I'm having sex with be a good friend, or have the potential to be one. While I prefer to have sex inside of a defined relationship, that's not necessary for me. I have found that I am capable of lusting after multiple people at the same time. While there are attention distribution decisions, I am perfectly happy to be in lust with not only Mulder, but Scully, Obi-Wan Kenobi, and Mr. Spock too.
I find that my continued attraction to people depends on their personality. I can say, "Oooh, she's hot!" upon seeing a particularly lovely woman, not knowing anything about her, but as I get to know people, their personalities begin to be factored into their attractiveness. Slackerprep is a particularly nice-looking young man. He also happens to be a pain in the arse. Although I once thought he might be hot, I now think he's exceptionally ugly. No lust. When I first saw pyrogenic at the Sidewalk Chalk Drawing activity, I thought he was the ugliest guy on the face of the planet. When I got to know him, I found him completely hot and lickable.
Where the people I lust after and my best friends intersect, those are the people I am in love with.
Practically speaking, I find it difficult to have the attention span for more than one man at a time, but I suspect that has quite a bit to do with some of the things I discuss mostly in the BDSM filter. I still have a deep fondness that could develop back into something quite strong where most of my ex-loves/lovers are concerned, but they're out of scope. I can be in love with a woman while I'm in love with a man. I was never so lucky to be able to test whether I could, practically speaking, fairly love more than one woman at the same time.
Recently I discovered something else interesting about my capacity for being in love. Several months ago, I realized all of a sudden that I was in love with somebody. This was a good friend who I'd known a long time, who I'd had the usual initial crush on, but it had faded out quickly after I discovered that the sexual orientations did so not align harmoniously. We went on to become good friends. Then I fell drastically, hard, in love. I freaked a little and didn't tell them. Within four days, they wound up in the hospital. After they were safely recovered, I no longer was in love; it settled back down to a background comfortable close friendship. My attachment evidently depends on need for it, as well as other factors.
So. I love.