Will was mean to him because secretly he desired everything Carlton had; intelligence, security, prospects.
Will could see 20 years into the future. He'd still be doing these silly hand slaps and wearing his outdated clothes in an outdated way, desperately trying to cling onto that spark he once had. The once cool Will Smith was fast becoming sad old man Will Smith. Left behind Will Smith.
Carlton would be working a cushy job, getting blown in his 00 Italian leather chair by his secretary and/or illegal immigrant maid twice a day. He could hire and fire 100 Will Smith's in a heartbeat.
You may have been the Fresh Prince but now you pay tribute to me, KING Carlton Banks. Never forget it.