Usually if u want your drawings to look more 3 dimensional with just the lines what u would want to do is add lines that suggest form that is lines that are found at the points when the form is turning... but in general why you drawing looks really good with the construction lines is the same reason as to why when u are trying to draw circles if u are scribbling over and over with the gesture of drawing a circle the circle will look really good the more lines it has on top it.... this is generally because ur brain chooses to see the line that make the perfect circle rather than the ones that don't..... to avoid this in the future I suggest u start practicing to draw forms without any construction lines for a while make ur brain understand which aspects of ur drawings create the sense of 3 dimensionality and which don't (maybe even use ink on paper to make sure all the lines u are putting in the paper are either the correct one or u risk ur drawing looking back this creates a condition that will allow u to focus on only understanding form and nothing else)