silversaab
The party with the greatest number of seats always gets an opportunity to form a government but if it fails to have the confidence of the house it is incumbent on Parliament to try to form a working agreement between parties to govern.

Jessica Murray
technically, the existing prime minister gets the first chance. so, under the rules of the system, if harper were to come in second, or even third, he would have first crack at forming a government. but, he needs to get the confidence of the house, first. if he fails, then the governor general needs to call on the party "most likely to gain the confidence of the house" - and that's going to be hard to figure out, although it will almost certainly be the party with the most seats.

the elder trudeau won the popular vote, but he came in second in the seat count. he *did* defer to the party with the largest seat count, although the political calculations at the time also suggested it was smarter to let clark hang himself than it was to govern in a coalition with the ndp. and, if you look at the situation today, that's also what the younger trudeau is really saying - it's not in his interests to place mulcair in power, he'd rather let the conservatives sit in a minority and hope the situation turns around in his favour.

www.cbc.ca/news/politics/canada-election-2015-justin-trudeau-interview-peter-mansbridge-1.3218597