Some of the common reasons why you may be inadmissible are:
If you provided wrong/misleading or withheld any information in your application.
If you are a security risk to the country by engaging in espionage, terrorism, violence, conspiracy, subversion or being associated to any group who is involved in such activities.
If you have any ties with organized crime (smuggling, money laundering) war crime, or human rights violations.
If you have been convicted of a crime outside Canada which is punishable for a term of at least 10 years.
You might be a permanent resident, temporary resident worker, international student or a visitor and still be denied entry into Canada if you are inadmissible. You might still have options in order to enter Canada; this
can be done by way of rehabilitation.