Snape is the only death eater able to produce a full patronus which had a definite shape(a doe). Why the other death eaters can't produce a full patronus? I read it in HP Wikipedia.

Because Snape wasn't a real Death Eater! OK, he was a Death Eater at the beginning, but after Voldemort killed Lily, he came on Dumbledore's side and he saved almost everyone he could.

He could produced a Patronus because he knew what love was. Love is the most powerful positive emotion ever (no matter if that love is painful or not). Therefore, Severus was able to produce it.
Honestly, can you imagine Lucius Malfoy or Bellatrix Lestrange producing one ?! They just mean it when it comes to make suffer.

Severus can conjure a patrounus because he's not really a deatheater. Deatheaters can't conjure a patrounus (I assume) because they have no good in them, no happiness, and no love. Only greed and ambition.

He's considered the only Death Eater capable of having emotions and happy memories.(How ironic is that?!)

I think it was because, ironically, he knew what true love and happiness were. He, unlike the other Death Eaters had emotions and happy memories.

Yeah exactly LilyZ~!
Also i reckon cuz Lily was a source of happy memory in alot of ways while i think the Death Eaters on Voldemorte's side in some ways lost all their true positive-happy feelings yknow...

To produce a full Patronus you have to full your mind with happy things, with the persons you love, the things that makes yous happy. Severus thought about Lily but the other Death Eaters didn´t had nothing "happy" to think about.
Dead?
Voldemort?
NOT HAPPY

I think he waz a death eater cuz in hp 4 serious tells Harry tat a death nevever quits being one...
Bt the reason he could produse a patronous waz becaz he had happy memories whereas the other death eaters happy memories were clouded by fear of voldemort and cruelness.they could hav had good memories bt tat would hav been of torturing or killing someone so they really cnt be happy memories...

well, they cant think of something happy.. that's all.. patronuses are for good hearts..

Because the death eater doesn't know what's real happiness their whole life they have been killing people and they live in fear cuz of Voldemort xD

Severus could produce a patronus because he loves. When producing a patronus one must think of something which brings a happy feeling. Death Eaters do not understand joy and happiness. Dumbledore said " Severus is no more a death eater than I am"