Anticipation.
When you like someone and starts wearing your heart on your sleeves, subconsciously, part of you expects that your feelings will be reciprocated. In love, courage is crucial yet not a lot of people is courageous enough to be selfless and brace the path of uncertainty.
We anticipate the worst and we ran away because we are scared to get hurt. How many relationships ended just because of this? While loving sends out the feeling of euphoria, the mental and emotional agitation is too much to take.