There are a number of reasons why South Sudan declared its independence from Sudan, but much of it relates to cultural, ethnic, and religious differences between North and South. The South is majority African, and significantly Christian, while the Northern population is majority Muslim and Arab. Many expatriate South Sudanese, as detailed by The Guardian, see this difference as one of the key reasons that a split was necessary; of course, I'm sure years of abuses at the hands of the northern and predominantly Arab Janjaweed militias couldn't have anything to do with that.
Creating a new nation isn't an easy thing to do though, and every new nation has challenges, as our own nation has proven. The various ethnic groups within South Sudan are themselves divided, and there has been unrest particularly in the nation's oil-rich areas, which in turn were the focus of the conflict within Sudan pre-independence. Furthermore, the South is totally dependent on Northern infrastructure to allow its oil industry to thrive, which will require an uneasy truce between South Sudan and their Northern neighbors. And, sadly, not all oil-rich nations are able to leverage that into real growth and wealth, as states such as Nigeria have proven. The rentier state trap, by which nations derive income mainly via selling off their resources to other states but rarely manage to distribute the gains of this process in any coherent or helpful wait, is definitely a danger.
Even if South Sudan is able to avoid the traps that other petrostates have fallen into, it must still deal with its own ethnic divisions, ongoing enmity with the north, and its disputed border with Sudan. There's also the matter of actually building a state. Institutions such as a functioning legal and justice system, for example, don't exactly spring up fully formed out of the clay. National identities cannot be forged overnight (as a look at American history can show easily). Nothing about South Sudan is guaranteed right now, which is not exactly an encouraging thing.
For the time being, though, the United Nations has 193 members. With luck, hostilities between Sudan and South Sudan will fade away. But even if that happens, there is no guarantee that South Sudan will be a successful state anytime soon. I wish them the best of luck of course, but I can't help but think that it is going to be a very long and difficult road for them.