This list was made by loads of people on the BBS, and was started by Blamitality.
With his permission it can now be viewed forever, regardless if the thread is lost. Thanks to all.

1.) You can use cum to put broken things back together. (Blamitality)
2.) For pleasure.
3.) Because you're single. (TurtleJuice)
4.) Good way to kill some time.
5.) Helps you sleep. (DekapitatoR)
6.) Because nobody loves you. (Sidorio)
7.) Make the most out of your bathroom.
8.) Avoid relatives. (paintballr)
9.) Helps you chill.
10.) It pisses off religous people.
11.) Annoys people who are to embarassed to admit that they do it too. (KingRugger)
12.) You can put the result on toast. (Kiddmeizter)
13.) Cum is surprisingly tasty. (Sidorio)
14.) It's goopy and fun to wipe on myself. (CHRIST) [Smartass.]
15.) Because its cheaper and more convenient. And you don't have to have a leech on your wallet. (Brick-Top)
16.) Gay guys can get kids. (The-Hydra-of-Spore)
17.) Because it's smexy. (Nicko9y)
18.) You can cum on people and make them melt. (SmellyCommandConsole)
19.) It's healthy! (spite-skater)
20.) to release sexual tension that may result in cousin fucking :0 (mariomusicmaker1)
21.) No risk of AIDS. (X-Gary-Gigax-X)
22.) Cum glows in the dark. (Blamitality)
23.) Strengthens the wrist.
24.) Makes you less social.
25.) Makes you think that real sex feels like a hand gripping your penis.
26.) It tastes like watery salt.
27.) You can close your eyes and be doing anyone you want.
28.) It doesn't have to be with just your hand.
29.) Measure cumshots.
30.) Helps you practice calling someone a wanker with your hand.
31.) Because Tramps does it. (Tramps)
32.) Jesus wants you to. (Sidorio)
33.) Keeps the Jews away. (Adam)
34.) Because its hip. (Jon4life)
35.) All the cool kids are doing it.
36.) COZ IT MAKES U SUCH A REBUL!!!!!1
37.) You can stare at your own penis without appearing too gay. (Sidorio)
38.) Because it's the closest you will ever get to having sex with anyone because you're a hideous deformed loser with a two inch dick that's bent upward and you have AIDS. (Kemcab, WTF?)
39.) It builds character! (FineAsFaux)
40.) Essential for cats' health. (munchies)
41.) You can make topics about doing it on NG. (Orange-Jews)
42.) Its a Cheap Workout? (Xerphox) ZOMG 42
43.) It's habit. (FineAsFaux)
44.) So you can have competitions with your friends to see who can ejaculate the furthest. (Neo-13)
45.) "Masturbating is shaking hands with the devil." (Tribalfusion-X)
46.) I can prove to people nothing happens when sinning. (Sinvader)
47.) A tasty snack in a jam. (Ceti
48.) Something to do in your spare time.
49.) To feel better about yourself.
50.) Self-learning.
51.) More interest to sex. (Lost-Wisdom)
52.) You can stare at your penis for 10 minutes and then say: lol. (disorderly1)
53.) Because the majority of Newgrounds does it. (Archkronos)
54.) Because you believe excessive wanking will make your dick bigger. (Vrael)
55.) You can cum at your own pace. (Fox)
56.) Because cum feels warm on your tummy. (HairyHungarian)
57.) Great way to feed animals. (Duffi)
58.) Cum can be mixed with Kool-Aid for a delicious drink. (Blamitality)
59.) It enlarges this thing called an ePenis. (Archkronos)
60.) Because Internet users are never going to get any pussy in real life. (TehreTard)
61.) Britney Spears. (Sugary-Cupcakes)
62.) Great way to break awkward silences. (Cosmonautical)
63.) Cheaper than a hooker and there's no legality issues if it yells rape. (Vrael)
64.) Because a hippo will bite it off otherwise. (Girouette)
65.) If a bitch wants to give you a lap dance and you do not have the money... then wank. (JoeIsCooler)
66.) Dogs do it and all men wanna be like dogs. (Vrael)
67.) IT TASTES LIKE CHICKEN. (jjrmet)
68.) God didnt give us TWO hands for no reason.
69.) To keep porn stars in business. (bobsmovie)
70.) It shows you're going through puberty. (Blamitality)
71.) Makes erections non-pointless. (Sidorio)
72.) No one else can do it right. (Charon)
73.) It kills kittens, and we hate kittens. (Satan0666)
74.) You can do it anytime anywhere
75.) Sometimes forces you to use your imagination
76.) Makes it possible for man to get dirt on any other male in the world
77.) After 10 years it'll still feel great
78.) If caught finish up and you'll feel better anyway.
79.) Can get you 50 bucks (sperm bank!) (Vrael)
80.) Otherwise internet porn is useless.
81.) Best way to use lotion. (Fox)
82.) So you don't wake up in the middle of the night with sperm-filled boxers. (Grade-Zero)
83.) Better than sex.
84.) better than Chocolate.
85.) Better than sex and chocolate at the same time. (Blamitality)
86.) Keeps your arm strong. (H-A-X-O-R-Z)
87.) Makes you feel ready to take on the biggest of tasks. (Sqrage)
88.) Makes God cry. (Javadoq)
89.) Gives you an excuse to call your penis a bitch. (HOLY-FUCKING-SHIT)
90.) Loss of protein forces you to get hungry and eat something delicious. (Blamitality)
91.) It's been done for millenia so don't resist the urge. (Vrael)
92.) You're gonna sin anyway.
93.) Releases Testosterone.
94.) You can easily do it multiple times in one day.
95.) Easy to do.
96.) The learning curve is 0-30 minutes.
97.) The more you do it, the longer it takes to climax (good for when you get laid.)
98.) You've always wanted to cum in your girlfriend's face.
99.) You're a guy. (Blamitality)
100.) It is impossible to make something epic enough for number 100.