I'd like to address this question to encryption algorithms in general, but, just for the moment taking DES for instance.
When studying the DES algorithm, we're shown the structure and how the algorithm works with some mathematical processing. To a newcomer in cryptography, this random amount of permutation, substitutions and xor really looked like arbitrary operations ordered in a complex way and seem a lot more... random. For all the substitution and permutations being done orderly, is there a concept or goal going on at the back of the algorithm designer's or is this just some sequential steps of random computation that makes decipher a challenging task?
The encryption algorithm has been chosen carefully. The S-boxes seem to have no weaknesses. Moreover, decryption can be done by the same algorithm applying the steps in reverse order.