It is quite easy to see that Eumenes would not have wished Perseus to win the war and become absolute master of Greece.
For, apart from their inherited dislike and hostility, the fact that they ruled over subjects of the same nation was sufficient to create between them distrust and jealousy and in general the strongest antipathy.
The only object they could have had, then, was to deceive and trick each other by secret intrigues, and this is what they both were doing.
For as he saw that Perseus was in an evil case, hemmed in on all sides, and ready to accept any terms in order to get peace, each year sending messages to the Roman commanders for this purpose; as the Romans likewise were in extreme difficulties, having up to the campaign of Aemilius Paullus made no progress in the war; and as the Aetolians were in a state of unrest: Eumenes thought it was by no means impossible that the Romans would consent to bring the war to a conclusion and make peace; and he considered that he himself was the person best fitted to mediate in the matter and reconcile the two adversaries.
Making these reflections to himself he had in the previous year sounded Perseus through Cydas the Cretan as to how much he was willing to pay for the hope of his services.
Walbank Commentary