I'm not sure there is a single "right reason" for conversion since people's reasons are often complex. I don't think it is necessarily wrong to convert for a significant other, as long as there is: (a) belief (on some level), and (b) commitment. The reason why I say this is that no one is going to have a fully formed belief as a Muslim without actually being a Muslim (just as no one is going to have a fully formed belief as a Catholic without actually being a Catholic). However one should at least agree with the general tenets (such as belief in God, belief in the Prophet, belief in the Qur'an as divine revelation, belief in the Hereafter) and have the willingness or desire to make a leap of faith, and to develop that faith.

In this case, assuming that the end goal is marriage, one should also have the desire to have a Muslim family and raise one's children as Muslims (which may or may not be what you want for the rest of your life).

However, there are no guarantees regarding marriage or relationships, and a good rule of thumb here would be to ask yourself, what if the relationship/marriage didn't work out - would you still be committed to being a Muslim? If the answer is "yes" then it is a good sign you are on the right track (even if a primary reason for converting is your partner); if the answer is "no", that warrants more consideration.

However, what concerns me about this question is that you say you are feeling pressured. This suggests that somewhere inside you don't want to do it or aren't ready. If you aren't ready, you aren't ready, and you shouldn't push yourself. Assuming you are in a Muslim-minority country, living as a Muslim and as a minority can be challenging, and if you don't have full commitment, it can be a difficult to manage that challenge. 

Anyway, things in life usually happen when they are ready to happen, and not before that. So maybe it is good to give it some time now and do prayer, reflection, and more research about Islam. (Even if you have already researched, there isn't a limit to how much one can research!) I am sure you will come to a decision when you are ready.

Also perhaps it is good to ask your partner to give you some space and not to push you, since this is a decision that affects your life and future and so it is one that you should be certain about.

From my observation, when someone converts to Islam specifically for the sake of a partner, the dynamics can sometimes get a little weird since their partner often becomes their first teacher about or model of Islam. That can set up a very slanted power dynamic and a loss of self (especially if it leads to their partner dictating to them a new identity and a set of instructions on what they can or cannot do in life). That, in turn, doesn't lead to the happiest of marriages. Anyway, I am not saying that is automatically going to happen, but it's another thing that you could keep an eye on if that is a factor here.