Short answer: In the way some verbs (as the verb "to love") are used in Portuguese (and other languages such as Spanish), the verb must be said/written followed by the "a" + the person/object that is loved. If you don't use the "a" it just feels weird; as if you were just saying "I love. My wife." Once you start thinking in Portuguese without even noticing, then it will start making sence. In the meantime, try to learn sentences as a whole until it feels natural. It helps a lot!