why do soldiers desert? and why are they applying for refugee status in countires like Canada, rather than fight in a war?

The UNHCR Handbook on Procedures & Criteria for Determining Refugee Status comments that while army desertion is not generally grounds for asylum, there are some who have valid claims. These include:

(from para 170).. there are cases where the necessity to perform military service may be the sole ground for a claim to refugee status, i.e. when a person can show that the performance of military service would have required his participation in military action contrary to his genuine political, religious or moral convictions, or to valid reasons of conscience.

171. Not every conviction, genuine though it may be, will constitute a sufficient reason for claiming refugee status after desertion or draft-evasion. It is not enough for a person to be in disagreement with his government regarding the political justification for a particular military action. Where, however, the type of military action, with which an individual does not wish to be associated, is condemned by the international community as contrary to basic rules of human conduct, punishment for desertion or draft-evasion could, in the light of all other requirements of the definition, in itself be regarded as persecution.

172. Refusal to perform military service may also be based on religious convictions. If an applicant is able to show that his religious convictions are genuine, and that such convictions are
not taken into account by the authorities of his country in requiring him to perform military service, he may be able to establish a claim to refugee status. Such a claim would, of course, be supported by any additional indications that the applicant or his family may have encountered difficulties due to their religious convictions. ..