Yeah its true that we can easily build trust with someone in relationship rather than to maintain trust whether in love or in friendship. It takes years to build trust and it can take a few moments to break someone trust, and sometimes it takes whole life to repair it but you can't build trust again.
In a friendship I don't think that respect and trust are two main things. Whether you don't respect your friend when you are only with him/her, some times you make fun of them and sometimes they make fun of you. But do you know what?? They think that you will never disrespect them in front of others, if you disrespect them or do some other bad things then you are gonna break the trust and gonna lose your friend.
In a relationship trust is the vital thing specially when you have already break them but they still with you. At this stage when your partner is already broken your single bad move can ruin your relationship and hurting them again will be the cause of losing trust forever. So always try to maintain your trust level by making them feel special. It's up to you how you make them feel special.