I could post countless quotes about trust here that might put things in to some context but I'll go with this one that seems quite fitting..

'Trust takes years to build, seconds to break and forever to repair'

In that quote you have the essence of the trust system and how it does and should work. People with low ranks don't often have positive trust because they have not been around long enough to build up that trust in them. Low ranks often have negative trust because it takes only seconds for them to break someone's trust, just like it takes them only seconds to make their many low ranked accounts. Once they're tagged with negative trust it's almost impossible for them to come back from that, hence the forever to repair.

Do not worry about trust in the forum unless you are transacting with others, if you are then be trustworthy and with time you will develop a reputation for doing so but one must question your motives for having a positive trust rating. If you are always trust worthy then your reputation will come soon enough and you won't need a trust rating for that.