to trust is to "allow without fear"
its so intangible and important.
fear is natural and built into our genes because it has proved itself to be necessary for survival. homo sapiens has fear as one of its self-protective mechanisms, but this mechanism becomes a double-edged sword. just like any other personality trait and physiological adaptation.

so when one lets down his/her guard enough and trusts, it gives an opening for hurt. it makes one vulnerable. in more sense of the word than one.
a businessman knows not to confide too much in someone else of its particular route to success, its business management and financial issues.
a thief knows not to tell someone else the method he/she uses to pick locks/pockets.

worriers tend not to trust as well. they over-fear in moments of uncertainty. they become especially vulnerable with people they trust in their moments of uncertainty. especially in their mind, they tend to treat uncertainty as a negative outcome. in other words, they become pessimistic. they are unable to have confidence in themselves that they are able to triumph in the uncertain situation, they fear as everyone does, that things will turn out to be disappointing. so how can they trust that everything will be fine?

trust is to have unwavering faith as well. a form of promise which is solemn and given after deliberation.
trust comes with faith and it becomes an important gift. its made all the more important, because its not given to just anyone. its given only after careful consideration in the conscious and in the subconscious. i'm sorry i have fear. its part of me. i deliberated and gave my word. trust me.

Dr Joyce Brothers

The best proof of love is trust.