"Spark" is a term that I hear often in relation to dating. In my experience, it's been used many times in relation to why I'm not getting a second date. As in: "You're a nice guy, but I just didn't feel a spark. We can be friends, but I don't think we should go out again."
I've certainly felt many sparks on dates--in relationships that have worked (well, as much as they can for a guy who's in his 30s and single) and just as often in relationships that I can look back and see were doomed from the outset. The younger romantic in me saw them as sparks of love. The older, more experienced (and perhaps more cynical) dater in me tends to attribute the spark to lust, something that can fade very quickly.
A single female friend explained to me that a woman knows within the first fifteen minutes whether their date has the potential to go beyond friendship. If that spark isn't there immediately, it's not going to be there, she said. If it's there, then it's all systems go and you give it a shot.
Sparks, fireworks, stomach fluttering when she walks into the room--all of them are wonderful sensations. But they don't hold up a relationship. In my limited experience with long-term dating, you just don't maintain those sensations constantly. In a good relationship, I think they still happen, but it's certainly not something that you can depend upon to keep a relationship functional. There are cases where I fully understand knowing immediately if it's not going to work out with someone. But I've had dates that started poorly spark somewhere along the way. I've had cases where I liked someone and asked them for a second date despite the lack of spark, and the spark came later. If it's borderline--someone you like and get along with but aren't sure if there's anything more there, eliminating them as a possibility for the rest of your life based on 15 minutes seems like an odd choice.
Getting a single guy's opinion on dating may not be worth the bandwidth that the opinion is using. But that's the wonderful thing about blogs--the bandwidth is free (for me). :-) Don't get me wrong. I like sparks--I want sparks! But I don't think I can base a relationship's future solely on their existence or lack thereof. I don't think that sparks always develop immediately. Not to mention that it takes far more than a spark.