This is something I'm going to have to handle when I get to it in the ever impending beginner's guide I'm trying to scrap together for Lucario, so I'll have to address this eventually. Here's what I've got so far...
Primarily, Lucario's playstyle reminds me more of characters like Falcon. Now, I'll admit that I'm kinda borrowin' from Strong Bad here, but I mean to say that they both play a heavily punish-based defensive game. Both Lucario and Falcon are aggressive in that their options in neutral primarily spawn from imposing threat via position and movement, hence the heavy reliance on DD. Another example of somewhat how he is played can be found somewhere in yet another of SB's posts:
Ape Academy (Which even if you don't play DK, this is seriously worth the read.), specifically that you are always interacting with your opponent, be it with pressure from spacing, safe pokes (read as: dtlt almost exclusively), combo starters of varying degrees, grab from DD, etc that keeps your opponent from controlling the situation as easily until you can actually land a solid combo. This is, within reason, at least, why Lucario feels like such an aggressive character: he is incredibly engaging to play, and I don't think it is possible to effectively play passively with him.
Want a rather fun specific 0% combo opener? Try SHFFfair>aFP>...plenty of things, character dependent. Looooong stagger/punish game to follow.