‘Then, Aaron, arm thy heart, and fit thy thoughts,
To mount aloft with thy imperial mistress’
Love, Arron’s motive was for his emotional attachment towards Tamora to be accepted and openly welcomed. This is a worth while motive for Aaron, as Tamora is the queen therefore has to power to make decisions out of the range of others. If Aaron builds a close relationship with Tamora he will be able to manipulate her into making decisions in his own interest. This may be his motive but it is unknown what his real, serious motives were, all predictions are just hypothetical.