Trust but verify?

I have a question which might seem silly to some. Apologies in advance. I sometimes read the phrase "Trust but verify" here on TAM. My question is, why should we? If we cannot trust our partners 100% (for me, verification means less than complete trust, I might be wrong though), then should we continue the relationship at all? I for sure wouldn't want to spend a single day (again) in a relationship where I cannot trust my partner 100%. My sincere apologies if I misunderstood the phrase. Thanks for pitching in.

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