To answer the first question, Ness was bored one day, and so he pestered Poo to teach him how to summon stars to rain on cities and thus destroy them. >>
(Honestly, I dunno why.)
Now for the other two questions. I much prefer Ness's Starstorm to Lucas's. I'd explain why, but Dajayman already did so for me:
Both the way Ness says "PK STARSTOOOORRM!" and the way he uses it are SOOOO much better than Lucas and how he says "PK Staaarstorm..." Ness's also seems more powerful to me.
If I
could change it, it'd definitely be Psi Rockin'
Omega over anything else. How it'd work...
There would be a small, invisible target area about 10 feet above Ness (one jump from Ness). Three large shockwaves would push all enemies within it towards that target (not all the way) and do 5-10% damage, each one progressively smaller so as to create the effect of a black hole (Their timing being similar to that of the first three sounds of Psi Rockin Omega). After those three inward-flowing shockwaves did there work, a vast amount of psychic energy would expand from the target area and create the effect of a Psychic Smart Bomb twice as big and twice as powerful as the regular one.
Once that damage is done, then *BAM* huge multi-colored explosion, opponents flying, happy victorious Ness.
...and yes, he'd freaking scream it. Scream it as if it was the last move he ever made. =D