You're not a specialist. Not trained for that. Coulson's telling you to make friends, get people in HYDRA to trust you, people you'll eventually turn right around and betray. You think they're all true believers? You think every person that works for HYDRA chose it? Can you really say you'll be able to turn on them when the time comes.
Speaking from experiance, its not easy.
And you do realise what they were getting you to do right, those triggers. How long before its you in that machine?