The Torah talks about the obligations of one who takes upon himself to be a Nazir or a Nazarite. The Torah says: לְאָבִיו וּלְאִמּוֹ לְאָחִיו וּלְאַחֹתוֹ לֹא יִטַּמָּא לָהֶם בְּמֹתָם, “To his father or to his mother, to his brother or to his sister, he shall not contaminate himself to them upon their death…”. (Num. 6,7) This is strange since a Kohen must retain his holiness at all times yet he is permitted to come in contact and attend the funerals of these relatives. Why may the Kohen and not the Nazirite?
The “Sefer Hachinuch” which explains the Taryag Mitzvot (the 613 Mitzvot) makes an interesting distinction. These prohibitions were placed on the Kohen without consulting him if he is willing to accept them. They came upon him at birth when he was born a Kohen. He may not be strong enough to adhere to these restrictions. The Torah realizes the frailty and does not place upon him this stringent law. The Nazir, on the other hand, took these restrictions upon himself so he knows his abilities and must know that he can abide by them. He thus must obey these laws.
When committing oneself to certain obligations one must know that he can carry them out. He must think twice before he makes a commitment. It is regretful that at times in haste we promise things that obligate us to carry out tasks or promises that we find later they are too difficult for us to fulfil.