You have already known from grammar course that there is a range of suffixes in Russian which can make a pet form of words or, on the contrary, exaggerate their meaning. Let's look, why we use such form and when it is proper to be used.
As you know, the Russian language is rich in the means which help a speaker to express his subjective opinion. Two people speak about the same thing or fact but speak about it differently not expressing their own attitude. We use diminutive-hypocoristic suffixes to convey emotions, estimation of an object. When we use such suffixes, first of all, we want to express our own attitude to the fact, rather than just describe it.
We often use diminutive-hypocoristic suffixes to change names in order to express our positive emotions towards an object: sympathy and liking. For that purpose we usually use the suffixes: -очка [ochka], -ечка [yechka], -онька [on'ka], -нька [n'ka], -ушка [ushka], -юшка [yushka].
It is more or less clear with names, but why do we use these suffixes when we talk about ordinary objects like a bed or a tree? Diminutive suffixes are usually used to denote small size of an object, exaggarating - big size. It does not concern emotions and estimation.
Дерево - деревце (a small tree) [Derivo - derivtse]. Дом - домище (a big house) [Dom - damishche].
But even in this case we can also express our subjective opinion using this or that suffix. In other words, a house can be not to so big, but when we describe it, we try to convey our emotions, that's why we can exaggerate state of things.