There is a major dispute if aveilut is deoritta or derabbanan. The consensus is that it is derabbanan. 
An important aspect of mourning is doing teshuva. Anyone who doesn't mourner like chazal instructed is considered cruel.
It is improper to mourn a deceased one more than chazal instructed. However, for a talmid chacham it is permitted but still it is only permitted to cry over the death until 30 days and give eulogies until 12 months. 
Beginning of Aveilut
A person would have to mourn based on a source from one witness, even if it is secondhand, or even a non-Jew if he is speaking casually.
↑The geonim hold that the first day is deoritta, while the Tosfot hold that aveilut is completely derabbanan. Shulchan Aruch 398:1 holds that it is deoritta but the minhag cited in Shulchan Aruch 399:13 holds that it is derabbanan. Shach 398:2 writes that we hold it is derabbanan.
↑Rambam (Avel 13:12), Shulchan Aruch 394:6. See Birkei Yosef 395 who writes that it is inappropriate if a person doesn't cry even one tear during the first three days after the death of a relative unless he is holding back from crying because of marit ayin.