"The heart of her husband safely trusts her." (Proverbs 31:11)
Some people you can trust in safely, but do not put your trust in relationships that are untested. If you put too much trust in an untested relationship, you may find your heart broken. You are setting yourself up for disappointment. If you don't want to be hurt in life, only put your deep trust in those you can trust in safely.
The Bible says we are to guard our hearts, so we should not let our affections go out to just anybody. Yes, you have to trust someone, your heart has to rest somewhere, but choose carefully its resting place.
Disappointment comes when your expectations are not met. The people who are closest to you can disappoint you more readily because you have greater expectations of them. People you hardly know cannot disappoint you, because you have no expectations of them whatsoever.
Unrealistic expectations will hurt you. Measure the trust factor carefully before putting any weight of expectation on something or someone.
It is true the Word says, "love believes all things", so you try and believe the best of people at all times, but you hold that faith in place for the other person, not for yourself. To have your personal hope and trust in the hands of someone who is unstable and wavers in their decision-making, is to risk emotional turmoil. Your own personal emotional needs must be deposited in the heart of someone you can truly trust.
Unless you know the person well, don't make yourself vulnerable to them by feeding off their affections or promises. Do not take them in to your heart, or allow what they say to register or accrue emotional value, or you will pay the price of disappointment one day.
There will be very few people you will build sufficient depth of relationship with in whom your heart can safely trust.