It is not the intensity of trusting that increases or decreases, but rather the object of that trust: it is the value most people trust that increases or decreases. No matter how intensely people trust that gold is worth $1,400.00, this does not increase its value in a cent. However, if most people trust gold is worth $1,500.00 rather than $1,400.00, then it value increases in $100.00. It is not the trust itself, but rather the trusted value: before people can trust any value, there must be a value to trust.
With many securities, and money, the ideal value is fixed.
A $10 dollar note is worth a fixed amount of $10 dollars, if it is redeemed.
What changes is peoples' trust that the security will be redeemed for that fixed value.
As the risk of default increases, the value people are prepared to pay decreases, and that is because they have less trust in the issuer than they did before.
You can see this with company bonds and government debts, they each trade a different discount to face value, because of the different level of trust people have that they will be redeemed, and not defaulted on.
Trust (risk of default) determines the price that a fixed-value asset trades at.