Hmmm its an age-old dilemma that has plagued female-kind since the dawn of dating; why do guys lie? I wish I had a fast and simple answer for you, but I dont. Just like girls, when guys lie it is for a variety of reasons, almost all of which have to do with HIM and not the person he is lying to (a.k.a. YOU).
When a guys says, I didnt want to upset/hurt/disappoint you what he is really saying is I didnt want to deal with your justifiable anger towards me when you learned exactly how I upset/hurt/disappointed you, can you see the difference? The difference is in the root of the lie and in this case that root is definitely not that he didnt want to see you upset. If it were really that simple, if it were truly just a matter of not wanting to hurt you, he wouldnt have done whatever it is he did in the first place. No, he knows he messed up and doesnt want to face the music and that is why he lied. Its as simple as that!
Now lets look closer at your specific dilemma. You say he lies about trivial things and that this worries you because it makes you doubt that you can trust him with the more important issues. Good for you for seeing this irrefutable truth without my having to point it out. If he lies about something small then chances are very, very good that he will lie about something important. Bravo to you for seeing this on your own. As to these trivial things he lies about you need to ask yourself if they are really just trivial? When he tells you he is at work only to be out with the boys you may honestly think it is a small detail but you should be asking why he wouldnt just tell you, I want to be with my boys tonight. Im willing to bet that the reason he chooses to lie stems from past experience. Possibly you have reacted badly to his being honest about this in the past and have left him feeling that it is easier to lie (now its your turn to look honestly at your past behavior to see if this applies). While this does put some of the blame at your feet (i.e. do you have a history of over-reacting to trivial things?) in all honesty your negative/angry/hysterical reaction should not cause him to lie. A truly stand up guy would tell the truth and deal with your reactions, even daring to point out where YOU are wrong if necessary.
If youre sure youre a laid-back gal with no hang-ups about nights out with the boys or coffee with an ex than the problem may be rooted in his experiences with other girls. If this is the case talk it out with him. Tell him that you know he dealt with over-reactive girls in the past and that youre not like that. Tell him your prefer an unsettling truth to a lie any day and that if your reaction is a negative one you vow to handle it with maturity by talking about your feelings rather than screaming and freaking out on him. If his lies are really a matter of negative past experiences with other girls than this sort of talk might help. However there is one big reason for his lies that cant be fixed by your honesty and reassurances. The most heinous, and sadly very likely, reasons for his lying is that he is really and truly doing something wrong and doesnt want to own up to it. He wants to have his cake and eat it to and that is just wrong. If his lies are born out of a real need for deception you need to kick him to the curb now! A guy who lies because hes been burned in the past is a very different sort of beast than the guy who lies in order to burn you. Once you know which kind of lying-guy youre dealing with youll be better able to figure out what to do.