What causes muscle soreness?

Is it lactic acid buildup, or is is torn muscles repairing?  OK, I've been working out relatively intensely (P90X), since Thursday.  I'm not very sore at all and would expect to be.  If I'm not sore, is it because I'm not working out hard enough or with heavy enough weights?  I feel like I'm working out pretty hard--sweatting and struggling with last reps, etc.   I want to know if I should be using heavier weights or what?  Thanks in advance.

If you are reaching muscle failure in the prescribed rep range then it is heavy enough. 

Muscle soreness is caused from little micro tears in your muscles that are caused by lifting heavy weights, those tears are repaired by the proteins in your body which is how muscles get bigger. You will also get lactic acid build up in the muscles due to oxygen debt in the muscles your are working.

Muscle soreness isn't always an indicator you're working hard enough...you just may be eating enough protien to repair the damage or your body may be that fast at repairing itself. If you really want to be sore as an indicator.. lift heavier weights aiming for reps in the 6 - 10 range.

In my opinion its all about quality not quantity. You can keep using light weights just more reps. and most important is your form. I call it "muscle mind discipline". dont just do your reps to get them done, think about what muscle your using and slow down. It might sound crazy but  good form is key to proper weight trainning. keep up the good work..............