Trust is a must-have characteristic in any good relationship, be it romantic or business-related. If trust is not present in the relationship, then the levels of rapport and interest in your significant other will be low and get lower with every passing day. The absence of trust can cause many problems, sometimes it can even lead to cheating and ruin an otherwise perfectly functioning marriage.
If you feel that you can't talk to your partner about any kind of important issue, the whole point of the relationship may be put in jeopardy. For example, if you can't have a normal conversation with your partner, how are you going to be able to discuss important things, such as marriage, children of personal finances? These are all important things that you need to talk about.