Why does drafting airtight deals leave me assuming the worst in people?

You build airtight agreements because somewhere you learned that if you don't, someone will take advantage. That is not paranoia. That is a skill, earned through experience, and it has value. But notice where the habit has spread. You are no longer only protecting the deal — you are protecting yourself against disappointment itself, in every room, with every person, before they have done a thing. That is not vigilance anymore. That is a tax you pay on every relationship, whether or not it is owed. This is not a flaw in your character. It is a habit, built for a reason, and habits can be examined and rebuilt. If the weariness runs deeper than tiredness — if it does not lift — say so to someone you trust, or to a professional. That is sense, not weakness.