Prophet Muhammad SAW said there are three things in Islam which are not matters to be joked about, and the first of these is marriage.
Sunan of Abu-Dawood Hadith 2189, Narrated by Abu Hurayrah: The Prophet SAW said: ‘There are three things which, whether undertaken seriously or in jest, are treated as serious: Marriage, divorce and taking back a wife (after a divorce which is not final).’
When marriage is discussed between two people, they're establishing a life-plan with dreams and aspirations, and these form the foundation of their decision to move forwards with the proposal. They effectively put their heart on the line, and for someone to be deceptive in this is extremely inappropriate.
Furthermore, the Prophet SAW said that the angels distance themselves from the liar, and this evil trait is not a characteristic we desire from our candidates.
Abdullah bin Umar (RA) relates that Prophet SAW said: When a person lies, the angel distances himself from the liar by a mile." (Tirmidhi)