why are some people so irresponsible & has NO sense of URGENCY?!

my CB proj group mates r juz so irresponsible! i've already told them that i cant make it tmr before 2pm cuz i've make up class till then, m i so insignificant? i'm really freaked out with their action! just like that last presentation, i can't believe that they can actually read up the case that needed 4 presentation just before it. Despite i was so so tied up and busy with my mugging, i also spent the time in doing my part - prepared the "backbone" of the ppt, yet they took it so lightly! just cuz of the small % of the assignment? i think such thinking is really too unbearable! it's too calculative! their attitude is totally wrong! i cant stand it really!!! what should i do? sianz... not saying they are not nice, but seriously i think their attitude and way in doing things should change!