All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad, sallallaahu ‘alayhi wa sallam, is His slave and Messenger.
We can summarize the answer to your questions in the following points:
1- As you read in our Fataawa, celebrating birthdays is not permissible. However, it is not correct to say that something can only be forbidden if it is clearly stated in religious texts that it is forbidden. Rather, some things may be included in a general evidence, like the statement of the Prophet : "Any kind of intoxicant is considered alcohol and every kind of alcoholic drinks (intoxicants) are forbidden." [Muslim] All kinds of intoxicants are included in this statement even if it has different names at different times or in different places.
2- Feasts and celebrations are among the religious rites. Therefore, it is not permissible to celebrate what is not legislated. What proves that feasts are among the religious rites is the narration reported by Anas who said: "When the Prophet reached Madeenah, he noticed that the people of Madeenah used to celebrate two specific days. The Prophet asked them "Why are you celebrating these two days?" They answered, "We used to play and have fun on these days during the pre-Islamic period." The Prophet said: "Allaah has replaced these two days with something better, 'Eed Al-Fitr and 'Eed Al-Adh-Haa." [Ahmad and Abu Daawood] The interpreters of the Hadeeth stated that they celebrated these two days only because the weather was moderate at the times they celebrated them and not for any religious purpose. Yet, Islam abrogated them.
3- Celebrating birthdays is a tradition of non-Muslims, and it is a new matter for Muslims, and this is something that cannot be denied. Therefore, celebrating birthdays is religiously considered as imitating the non-Muslims. Even if we presume that this is not imitating the non-Muslims, the narration in point 2 is evidence that it is not permissible to celebrate birthdays. The scholars clarified that the pre-Islamic celebrations are evil in all cases.
4- Making someone happy is something urged in religion, but it should be with a permissible means. The things which you mentioned, like weddings and the like, are one of these permissible means. However, a wedding is not a (religious) feast, because a feast (‘Eed) linguistically refers to something which repeats at particular times while weddings are not so considering each person’s case individually. For more benefit, please refer to Fatwa 13538.
May Allaah enable you to do whatever He likes and pleases Him.
Allaah Knows best.