I thought the classifier of "心" is always "颗" or "个"? But why here is more common to use "条"?
Actually, IMO, both are fine, only the situations of which to use are slightly different.
For example, after pursuing a girl/woman for a long time with all the ways of pleasing her (gifts, flowers, money...) have been tried but still at no avail, your friend might say to you: "你死了這條心吧", which equals to "你死了這一條條的心機吧". It slightly advises you to adjust your mind (心態).
On the other hand, you are in a one-way love (單戀) situation, and it is obvious no hope to continue, your friend will then say to you: "你死了這颗(愛她的)心吧". Because only when the heart is dead, you may wipe her out of your mind (死心).
Further to the other answers, perhaps it might be useful to have a workable rule of thumb to decide when to use what in terms of the appropriate classifier?
- If the anatomical heart is referred to, it is 心脏, (脏 being the viscera) So, you'll say 心脏病, and not 心颗病 or 心条病 or 一颗心病 or 一条心病
However, if you merely say, 我的心, or 他的心有问题, it is understood to mean the anatomical heart, unless it is further clarified in context, or a follow-up statement to mean something else.
- If 心, however, does not mean the anatomical heart but is meant to represents or symbolizes some emotional, psychological condition or circumstances, then the "classifiers" of 颗 or 条 would normally be used as the anatomical heart is neither a "颗" nor a "条", anatomically speaking that is. Thus I submit that calling these classifiers as "classifiers" is a misnomer in this context.
It thus begs the question why say 颗 or 条 then?, and not, say, 片? I leave it to others more knowledgeable to answer. I would just give a personal take on this.
I believe 条 is used because this word when used with say 路, as in 同一条路, it means, metaphorically, having the same destiny or fate being "on the same road together" as it were. Thus 一条心 would be to have one single "piece" of "heart" jointly or in common ownership, signifying a common goal or purpose because the numeral "One", "一" also stands for "Unity" in Chinese; thus 一条心 would be "Unity of Purpose"
Finally, in terms of ancient Chinese understanding of human bio-emotions, they are narrowed down to five basic feelings that are each associated with a corresponding organ in the body:
Anger with the liver, Fear with the kidney, Joy with the heart, Sadness and grief with the lung, Worry with the spleen.
However, it is strange that the brain is completely left out. Maybe it is so obvious that intelligence is associated with the brain that it is superfluous to even mention it. And of course we all know what the sex organs represent, or do we?