Thought, Tact and Zeality: One thing for you to keep in mind. Love might be love, no matter how intense, but with pain it's not worth it. But ones you see how you can have as much fun as possible with it (suppose ya fall in love with someone who has a boyfriend but wouldn't care and still flirt in a such a way she'd enjoy, as well as you) that would be worth it indeed. 
I don't really mind that, since I'm not looking for any kind of relationship at the moment. But hey, fun makes ya happy! No fun and yer letting yerself go emo and pain strikes you right where ya live! I've experienced that.
You might find the book
The Gift of Pain to be an interesting read. It largely deals with the misconception that since pain is unpleasant that it is also bad.
But as for your general statement, "with pain it's [love] not worth it," I would argue that either the love
is worth it or it wasn't love. Love is inherently a communal affair; if you love someone and that love isn't returned, then that isn't really complete love; it is half-formed. That is another reason why I would say that love isn't an emotion. An individual feels an emotion, but with love, two people experience and share it. Therefore, if there is an excess of pain and it outweighs the "love," then that is a sign that the "love" isn't love at all.
For your example (flirting with a woman who is supposedly committed to someone else), I would point out what seems to be a flaw in your thinking. In your example, one falls in love first and then communicates (aka, flirts). Pish posh. First one communicates and then love grows between the two. Love from afar is not love, it is obsession with an image that exists only in an individual's mind. Flirting is carnal, it is physical and material. Sure, love might flirt, and but flirting isn't love nor is it necessarily an expression of love.
Additionally, I would strongly attempt to disillusion you as to the notion that love is "fun" or "happy." Love can include those things, and the best love will be quite fun and happy as a side effect, but take those emotions away and love still remains. If one takes fun and happiness away and discovers love isn't there, then it wasn't there in the first place. Love transcends such base concepts as fun and happiness; it will often be those things, but to say that love
is (that is, its nature is:) "fun" and "happy" is like saying that water is powerfully wet stuff -- we define the latter based on the former, not the other way around.
As for physical attraction, don't count on it. Just because it seems bad doesn't mean it it. Unconditional love also does at one point hold lust, and it's all a part of mutual enjoyment. With a man and woman, it starts with the eyes, and a warm welcoming smile, friendship, a cheerful laugh, a little fun, holding of hands, then back to the eyes, there she's to embrace you, and then a kiss, cuddle, and after that is where things spice up. Purely physical tendency often is rejected by those with higher moral value and self respect. But when you love someone truly, and have as much fun as possible, it's obvious you'd want to spend time with em more. And when the feeling goes stronger it tends to enter the physical enjoyment side. It ain't bad!
Has anyone said that physical attraction is bad? There seems to be a lot of "love transcends physical attraction," essentially stating that it is a greater thing that doesn't need the lesser, but I don't recall anyone saying it is bad.
And no, Unconditional love does not, at one point, hold lust. It can, but it doesn't always. The two aren't inclusive, but neither are they exclusive. The best example of this is the love of a (good) parent for their child; the love is unconditional, but good lord, lust is so very absent. If lust was present, even in the smallest part, that is what we call "perverted."
The problem with physical attraction and sex is that it is dessert but humans seem to try to make a meal out of it. Love is the meal, the meat and potatoes, where we get our sustenance, our vitality, our energy, etc, where life, conversation, and intimacy happens. Sex is the dessert; it is tasty, something to look forward to, maybe to have coffee over, but if you make a meal out of it, then you are missing out on all the nutrition you need to survive and be healthy.
Edit: Thought, whadya mean? First cometh the dream, then cometh the goal and then enterth the dawn of reality!
What do you mean, "whadya mean"? I've said a lot of things, some of which I am sure is in reality probably bunk, but you'll have to identify what the question is in reference to before I can tell you what I mean.
