When you first meet someone, and you get the feeling that you would like to get to know them better, you make a snap judgment about trust right from the start. You do not realize you are doing it, but you are. If you feel that you can not trust someone, that is going to dampen any hopes you may have of a relationship. If you feel that you can trust someone, you may start dating, but that is only the beginning of the equation. Trust should be given, but relationship trust has to be built over time. It can be the deciding factor in the longevity of any relationship.
There is always, or should always be, some relationship trust in the beginning. If you feel that you can not trust someone right from the start, there is probably a reason for that feeling. That is a sign to walk away right away. If you feel that they are trustworthy, they have to prove it to you time and time again, just as you will have to prove to them that you are trustworthy as well. Think of relationship trust as something that grows each and every day and that is the foundation for a great future. Without it, you have nothing. A good relationship is impossible without trust.
Some people think of relationship trust as knowing someone will not cheat you on with someone else. While this is a huge part of trust, there is simply more to it than that. You have to trust that a person will never lie to you and never do anything that will cause you pain. That covers a huge variety of things. Trust can be broken in many ways, but it can also be repaired, depending on the extent of the damage. A long term affair may deplete relationship trust forever, but lying about spending money on something can often be repaired. Most have a threshold of what they will endure and what will break the relationship in half for good.
Sometimes, the amount of relationship trust needed depends on how long you have been with someone. Those that have been with each other for a long period of time have a better chance of overcoming a breach of trust than those that have been dating for only a short time. Sometimes, though, the breach is harder on those that have been together for a long time because they have built a lot of relationship trust and the breach is too much to bear after so long together.
If there has been a breach of relationship trust your life, think long and hard about what you want to do about it. Some easily give up their marriage or long term partnership before they really think things through, and they later regret it. Give yourself and your partner some time. You will know if they are truly sorry or if they don't care. That will help you make the right choice. There are great books and plenty of therapists out there that can help you rebuild relationships if you wish, and some can end up being better than ever after a traumatic event.