when you act like something is your only option (very rarely is something ever truly your only option) it makes you appear desperate, which is off-putting.
for example when it comes to getting a date, if you act like it's your only opportunity, the girl is going to pick up on that desperation and be turned off (and also make her question "other people must not like him, why would I?"). It's basically giving the other person too much power by letting them know what a significant difference they could make in your life, which usually results in: 1) the person being scared away due to feeling responsible for your feelings; or 2) the person taking advantage of that power. either case, not an ideal scenario. Same applies to work, friendship, etc.
my approach is to always be conscious of the fact that you do have options (if you're certain of this yourself, people pick up on it), while not letting that scare you from choosing one of them