At our level, to have pure motivation is absolutely impossible... and to say that we have pure motivation would be a total lie. At this level, the extent of our motivation are all tainted with the 8 worldly concerns... although we may not feel it, but our subtly it is there... and if we look closer we will realize it. I believe one does not have pure motivation until one reaches 3rd level Bodhisattva, the path where one does not get dragged down to samsara's delusions anymore.
Until then, we are all tainted with the 8 worldly concerns. I think it is because of this that at our level, instead of forcing on a pure motivation, to have guru devotion is more important. It is through having this devotion that helps us collect greater merits, since our motivation in Dharma is unpure and the merits generated from that is very limited.
Something we should ponder about. I've seen many people treating Lamas very poorly... Some sponsors would actually show a very black face when their Guru tells them off etc. So, before we even think about having pure motivation, think if we're serving our Guru sincerely, the one person that gives us the Dharma.