Coincidence is a lazy way out of or into fictional situations. An ex-lover comes back into town after the main character is married and pregnant? A character loses their job only to have an opportunity to go on a vacation? Do you use coincidence to fix your plotting problems? Creating artificial motivation is transparent and a superficial fix.
Why you can’t get away with it:
Coincidence looks like thin plotting and lack of imagination to readers. Coincidence is obvious which makes readers feel that they are being played for fools. Readers will feel that the writer thought they could get away with coincidence instead of plotting more intricately or with more motivation/reason for character actions. Readers will lose loyalty to the character and author as you’ve lost readers’ investment in the outcome of the novel.