LaTeX forces a straight right margin for completed lines, so it will space your words out to achieve that. You can tell it to hyphenate words instead by inserting \- into any hyphenation locations in the word you want to break, e.g. this is my thesis and it will be finished soon. the topic of this the\-sis is very inter\-es\-ting.. You can also tell it to be a little sloppier with the sloppypar environment: \begin{sloppypar}this is my thesis and it will be finished soon. the topic of this thesis is very interesting.\end{sloppypar}
