Dominica said:
How can the visa officer say the relation is not genuine in spouse sponsorship
If there's not enough reasonable documentary evidence. It's not enough just to "say" that you guys love each other and that it's not a marriage of convenience. You have to prove it with different types of supporting documents, outlined in the application checklist and also the spouse questionnaire forms you can download from the CIC website. The more supporting documentation you provide, the better. They will consider all facts you present them with. Some of the factors that might flag your case as suspicious, for example, if you got married very soon after meeting each other, if you registered a civil marriage but didn't have a wedding ceremony with guests from both sides of the family, if your family don't know about your marriage, if you haven't lived together at all (a long-distance relationship) or only for a very short time, etc, etc.
The approach as to how to prove your marriage being bona fide depends on the particular specifics of your circumstances. Having one or more children in the marriage is usually considered the single most convincing factor.
If they suspect enough then they might call the applicant for a personal interview.